Their exposure is more than just their ownership of SpaceX.

If the SpaceX IPO bombs (or even merely underperforms), the expectations for the Anthropic/OpenAI IPOs collapse, and with that, everything else AI.

AI companies can't afford to let any AI company go down.

I'm not sure this is true for Google. Ignoring their equity in Anthropic, AI is generally a threat to Google, since it's the closest thing to upsetting their search monopoly. The best case for Google is if OpenAI and Anthropic are way out over their skis, and Google is the only major player left with their sturdier financial position. The worst case is if ChatGPT/Claude completely displace search and nobody wants to pay for gemini. I find it unlikely that all three go down for the same reason.