This is an unusual reason for purchasing Bitcoin because normally the reasons you'd be buying it would be if it represents good value and you're adding it to your portfolio of traditional assets as a long term investor, or if you want to day trade it, then large buys and sells may be something you'd analyze especially if its institutions that are making the purchases as this would be a positive indicator for growth
You are right, that is the major reason why we should consider buying bitcoin, but if you look at the investment part of it, the price of the coin really matters, this why op said we should also follow the whales to accumulate when they are doing so instead of joining some people to panic sell.
The time whales accumulate coins are when the market is dumping and some investors are panic selling their coins into the hands of these whale, so instead of us too panic selling, we can just simply join the whales to buy these coins and wait for it to pump again to take our profit. It is either we join the whales to accumulate or we sell the one we have to them. We have to choose one.