Actually, he's angry with OpenAI, which he co-founded. He tried to merge OpenAI with Tesla, and when he didn't get his way, left the company. He continued to collaborate with them and eventually opened his own AI company, xAI. He's suing OpenAI now, and is essentially at war with them. OpenAI joining with Microsoft, and now Apple, is a big hit to his plans to be a big player in AI. His tweets were little more than a temper tantrum. And I say that as a fan of his.
My thoughts as well, but stated better.