so you are not entirely correct on this, a newbie can start holding his first Bitcoin from a private wallet and not in centralized exchange.
No one is saying that you shouldn't store your long term bitcoin holdings in a self custody wallet but for a brand new investor who is a no coiner, he doesn't need to start learning about wallets when his discretionary income is available because that will delay him from getting start. You don't need anything that will delay you.
Wallet is the technical aspect of Bitcoin and a brand new investor might fight it difficult to understand it immediately, because it's not everyone that can catch up things fast. There's nothing wrong if the brand new investor starts buying Bitcoin from an exchange and keep the money there while, he continues to learn about which wallet will be his preferred one.
Don't forget that if you rush into using a wallet in the beginning, you might make the wrong choice because you don't know a noncustodial and open source wallet which will also put your bitcoin investment at risk. Just like some newbies who are using Trust Wallet and they will agree with you that it's a good Wallet.
To be clear, Trust Wallet is not a good wallet, since it is closed source and it is run by Binance.
With wallets it is better that they are open source and don't have any potential back doors that might be present when they are run by a third party.
Another reason of storing your bitcoin in an exchange in the beginning is to pile it up there till it reaches up to $500 and above before sending it to your self custody wallet to prevent you from paying high transaction fee in future if transaction fee becomes very high so that, you don't use almost all your profit to pay for transaction fee because of to many small small UTXO. It's good to have a good UTXO management as you are piling up your bitcoin for the future.
Honestly speaking with you whether these two funds are in place or not it can not stop or should not stop an investor from investing or accumulating if the discretionary income is available. And for your information the back up funds is not strong or useful as emergency funds because as the name implies "emergency" that is how useful it is and it is used when something unexpected happens and as an investor you don't pray for such things to happen even though you will prepare for it and fortunately it doesn't happen all the time, sometimes you can hold as many years as possible without any serious emergency.
Who told you that backup funds is not as strong as an emergency funds. Emergency funds is part of backup funds. Backup funds consists of emergency funds, reserve funds and float. That alone has shown that backup funds is stronger than only emergency funds because if you have your reserve funds and you are being hit with real life emergency, you can start using from your reserve funds without tampering your emergency funds. It's if your reserve funds isn't enough to tackle the problem that you can start dipping hands into your emergency funds.
For sure, reserve funds have more options as compared with emergency funds, since we could imply that if all that we have remaining in our back up funds is emergency funds, then that would imply that most, if not all, of any of the other funds have already been used up, so that the emergency funds would be the last level of back up funds that would be available prior to having to dip into any bitcoin that had been accumulated... and it seems that a bitcoin investor is trying to protect his bitcoin investment and to continue to build it, so if all other funds have been depleted, then the bitcoin investor would have no choice but to dip into his bitcoin holdings, which means that he had run out of other options... and which also may well mean that he may want to rebuild.. yet he is set back in terms of either ending up depleting or completely exhausting whatever bitcoin he had built up.
If a guy came into bitcoin, and he had goals to build the bitcoin for 4-10 years or longer, and perhaps he was even hoping to reach overaccumulation status after 2-3 cycles of investing into bitcoin and/or maintaining (holding) his bitcoin after building it up, yet if he prematurely taps into it, he might considerably delay his abilities to reach overaccumulation status or maybe he will never be able to reach it merely due to his own sloppiness in regards to how he had managed his back up funds and/or various funds he kept on hand in light of his own expectations of his present (or future) income and/or his present (or future) expenses. Guys might not exactly know the future, yet sometimes they engage in sloppy and/or reckless behaviors because they get greedy or they just don't adequately prepare themselves financially psychologically for a variety of scenarios.
UTXO management is both for the costs of the present transactions, including that exchanges will sometimes charge higher fees when moving coins from their exchanges in order to suck additional fees out of their users, and also it is important to have UTXO management options in the future, which could be quite problematic and expensive if a person has a whole bunch of UTXOs that might have had values of $50 or less, and then each UTXO might be expensive to transmit, and the problem will become even worse if there is a choice (or a need) to transact with large amounts and each UTXO ends up magnifying the fee, so historically there had been guys that had hundreds of small UTXOs, and then when they went to combine them all, they could have cost $100s of dollars to transmit.
In recent times (such as the last year or more) we have been experiencing quite low and affordable transaction fees, yet we cannot know that those low and affordable transaction fees are going to continue, since there can be either attacks on the chain or even unreasonable clutter that might cause fees to go up, so guys who have a whole bunch of small valued UTXOs might not realize how the fees work when they go to combine those UTXOs or if they want to send several of them at the same time, and they could have had avoided (or at least lessened such matter from becoming a problem) by keeping many of their UTXOs that are larger sized, such as $500 or more as Sim_card mentioned as a possible target minimum (self-selected) UTXO size.