It's technically not pumping and dumping because Elon posts memes that shill his investments but he rarely gives a call to action and if possible avoids naming the cryptocurrency in question.

He's definitively manipulating the market in the sense of the colloquial meaning of the word.

If there are public signals that predict the performance of a stock then those who benefit the most are those who execute their trades as quickly as possible and since Elon knows what he is going to "announce" on his Twitter account he is going to benefit massively.