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Because the alternative is that Grok are reselling OpenAI API access fraudulently.

OpenAI would know that they were doing this. Reselling like this is against their terms of service: https://openai.com/policies/terms-of-use

My assumption was that they simply have a contract with openai.

Where does it say that goes against their terms of service? I can’t see anything that would preclude Twitter from just plugging in the API and using it directly

I think it's far more likely they're building off some open source foundational models, rather than doing one from scratch OR using GPT under the hood. If it was the latter, don't you think we'd hear about how "Grok is on par with GPT"? But you don't, people think it's not as good (but funnier?)

Occam's razor probably