Terrance Tao has for a fact appeared in promotional material for OpenAI. Based on my Googling the consensus seems to be he is paid for it, but I cannot confirm that.

I do think it's very likely that OpenAI pays for solutions like these to put in the training set, and then we get material like this Reddit thread. They market themselves as selling "intelligence", and solving these math problems is something people view as highly intelligent. I'm not a mathematician, so I cannot fully judge it, but based on my experience using LLMs for novel problems in other domains, they seem to really struggle with things that aren't common. That leads me to believe they train for specific outcomes like this. Also, there are a lot of jobs out there for data annotation, including software problems (Meta has basically reorganized its entire engineering department to create training data for coding problems).

This comment on the Pelican svg better articulates what I'm getting at: https://news.ycombinator.com/item?id=48950883