I only consider the difference in the past one year, it looks significantly lower than in past 7 years. There is possibility that hoarding had started far back as last year June and the higher price this time could have reduced the acquisition from new investors. The addresses in 2016-2018 could also be from ICOs and new exchanges, also, just assumption. The bear market also slowed down the exchange, this might extend till next ATH

The number or graph shown on the tweet can't be deemed as accurate because nothing was mentioned about each of the addresses being unique or not.

There could be more or less people who potentially owns 1 BTC but i'd say more as others could be moving their BTC constantly(for profit) through trading, lending, investing, etc then buying it back to complete their 1 btc in the future.

i save all of my bitcoin on one address only but the good part is that i dont leave them idle for a long time , instead i use them or i cash out all of them on a certain time .  so far i dont any have bad experiences from hodling a bitcoin and from the online wallet provider that i use   but your suggestions that we must diversify our bitcoins to many wallets is good , just incase one wallet could be hack and we can still have a back up funds left to other wallets  .