You not seeing what's wrong with his tweet means that you already accepted that it was a factually true statement, which it is not. Humanity wasn't been held down by small-brained babies until the advent of the C-section.

Secondarily, what point do you think he's trying to make? I'd bet that it's about humanity being more intelligent now that these giant-brained babies have an alternative escape route. I'd bet all his ill-gained wealth that he was a C-section and he's also bragging about how intelligent he must be. One commenter also already mentioned that Musk is replying to a eugenics-pusher.

It's never a good idea to read something that lacks and requires credible citation and say, "I guess that sounds right." Intentionally or not, you often add that to some folder of "true stuff I read" in your brain and start repeating it back as fact when relevant.

Oh, and Musk is a vile fuckwit.