They are certified hodlers, not into rush and they know when to unload. It means that there are plenty of Bitcoins when FOMOs rush into market. Theres always buyers when those 10 million are unloaded.
They might be or maybe the owner of that is already dead, which I hope not, I mean that is just a speculation. As far as I know, these HODLers like to move their huge amount of bitcoin from wallet to wallet, and they don't usually put all of it in one wallet, they cut it into pieces and manage with different wallets. that is 10 million bitcoins, almost half of the total supply.
Much more scarier is the fact that this too high of an amount didn't circulate in the market which is bad for a currency in the rise like bitcoin. Hopefully things change for the better on this coming halving because if it doesn't end well bitcoin might collapse, which will pretty much mean certain death for the cryptocurrency world.