Like the pyramids made by aliens ridiculous theory. At this point literally half his tweets are just conspiracy non sense.

I do believe we have already established it was mammoths - there was a thread on that just a few days ago. ;)

Can you point me where he actually promoted that theory? Or is it just some meme?

It was a joke. Autistic people fail to comprehend sarcasm and humour.

https://twitter.com/elonmusk/status/1289051795763769345

Further down in the thread, he linked to a BBC article about how the pyramids were built by human workers.

I think plenty of autists can understand sarcasm and humour, specially that. It's more-so <70 IQ that can't.