They could also do it at any other time to increase their bitcoin holdings, then.
In my view their interest, which aligns with that of all other major owners of crypto, is to acquire as much real currency as possible.
Acquiring more crypto by decreasing its valuation is not furthering that.
Maintaining a high valuation and positive sentiment in order to convert buyer's real currency into profit to cash out is what I believe their strategy is.
Presumably the hypothetical price change is reversible or there would be no point. So lower price, buy, raise price, sell, repeat. A dump-n-pump.