he doesn't need to pull it all out, he can always take out a personal line-of-credit borrowing against his Tesla stock. He has access to the full $200billion without having to sell any shares.
(he seemingly has access to a lot more money, too -- Tesla's full $1T market cap -- since he's the unilateral decision maker there. He can just buy up bitcoin under Tesla's name to boost their bottom line.... and it works)
Selling 10% of the Tesla stock is definitely a way to sneak some liquidity out of there (b/c liquid cash is always nice), but it's also most likely a performative stunt to remind people he's *legally* not paying taxes on his net worth && pretending he'd be willing to pay taxes if he made his money "the old fashioned way"