I'm just wondering why they sold compute to Anthropic if they were planning on still competing in this race?

Competing doesn't mean winning

The rental deal can be terminated by either side with 90 days notice, and presumably Musk would do so if he needed the compute or generally thought it advantageous to do so. For now he doesn't need the compute.

The rental deal may also have been at least in part to juice the SpaceX IPO and to help Anthropic stick it to his enemy OpenAI.

As a point of comparison: Samsung has sold smartphone chips and later OLED displays to Apple for over fifteen years.

Deals like this that look awkward from the outside but are mutually beneficial to both participants exist everywhere.

Likely because they had the capacity to spare. Prior to Grok 4.5, I doubt there was much demand for their models.

The revenue was critical to making their IPO numbers look a bit less insane.

Timing. They had a massive amount of compute coming online and a serious pipeline of more arriving.

a lot of that compute is used for inference, which is demand-based

For distillation deals lol