Isn't it possible they're dumping the rest of their holdings?

That or they are certain they will get burned sooner or later, and therefore even if they tank the value of their holdings it's at a time of their choosing and with an amount that's predictable.

Contrast with the scenario where they keep doing it, steadily increasing their bitcoin holdings and even seeing the proportion of sales using bitcoin increase… and THEN hit a wall, not of their own choosing, and see the value collapse.

Even if they are not dumping their holdings at all, there's still a case to be made for pulling a plug at a time they control, over an amount they control. If it collapses, they can put a hard limit on how much they lose: that being, the previous value of their entire bitcoin holdings.

The statement in the tweet said that they won't sell.

That is as amendable as the policy of accepting payment in bitcoin was - and right now, with Tesla having a nontrivial position in bitcoin and with Musk being an influencer of bitcoin price, it is in the company's interest for him to say that it is not going to sell, even if it reverses that policy tomorrow.

To buy more at a lower price?