And aside from KYC issues, another recently posted article pointed out that when a single individual tried to unload around a 1500 BTC (%0.03 of the coins in circulation at the time) it caused a liquidity crisis that crashed the market by 20%.[1]
In this case the individual in the Twitter thread has an amount of ETH of that magnitude.
Basically because crypto is performing the job of a speculative asset it results in the liquidity issues that you would see in things like company stock.
[1]: https://blog.dshr.org/2022/02/ee380-talk.html?m=1 (see footnote 11. The gist is that this sale put enough pressure on the order books that a lot of leverage positions cascaded into selling as well causing the flash crash)