I wonder why Apple stayed absent from signing this?

I wonder if it has anything to do with Apple’s legal case against OAI.

This would support the hypothesis that they are incentivized as compute providers and not simply because they're losing the frontier llm race

Maybe less shocking, but still, so is google. I know they are one of the labs, but that being said they rely far less on gemini for revenue as opposed to the big 2.

I think Google can play both sides here: they may end up a frontier lab if OpenAI/Anthropic dissolve, or a regular LLM provider, in which case they could be serving open models.

I think if anyone at Apple signs a plea that has the word 'open' in it they spontaneously combust into flames

If it doesn't involve a walled garden subject to Apple's domination and control, it's not an Apple product.

Apple is allergic to industry-spanning consortiums.