Nobody knows which coins Satoshi owns, it's just a guess for the very early coins and that guess gets progressively less accurate as time goes by. And this was a long time ago. There was no particular reason to think back then that Satoshi spending his coins would tank the price. Everyone back then was spending Bitcoins because that was the only way to build the economy. The idea that if his coins move everyone would panic is a post-2015 idea when Blockstream killed Bitcoin as a genuine means of exchange and it became all about sitting on them as a speculative "investment".

But if he did want to spend he could just start from his last coins backwards.

He could tank the price to $1000 and still be a billionaire. There's a much more plausible reason for why Satoshi's coins haven't moved.