I've always just assumed Musk knew something we didn't know. About a stealth project or startup that already developed some form of what could be described as 'general intelligence'.

If not, if he just knows what AI professionals in the field today know, and such stealth project doesn't exist, then I really do think he's worrying about "overpopulation on Mars" as Ng put it. Because there are zero indications in publicly available AI research that we are even close to developing AI that can think for itself, we can't even describe how that would work in theory with any certainty (beyond guessing), let alone implement it.