I don't know if he's going to use the advantage of selling everytime Bitcoin pump beyond $40k or so but I think Elon Musk is considering of holding the investment for a long-term plan than make it more worst than before to sold everything in one go.
Everyone is free to sell their Bitcoin, including Elon Musk. Moreover, he includes whales who have very large capital, he can do whatever he wants.
I agree with you, but the way that Elon Musk is doing is wrong by manipulating for personal gain. This ends up hurting a lot of people, this is
what ultimately makes Elon Musk look ugly in the eyes of the crypto community. But I doubt that Elon Musk is one of the holders who plan to invest
in Bitcoin in the long term, because if he is, Elon Musk will not deploy FUD that makes Bitcoin price crash. This is evidence that Elon Musk has already
sold his Bitcoins when Bitcoin reached the ATH price, and now it looks like Elon Musk is trying to collect Bitcoin again when the price is already low.