Global Illumination appears to have been working on a browser-based multiplayer Minecraft clone (https://www.biomes.gg/), not anything explicitly related to AI content generation.

Odd acquisition choice unless OpenAI is going to do an unexpected pivot.

There was that Sims post the other day from a16z as well. Seems like a plausible next area for them to take on.

Was it them or someone else who trained their bots on Dota a few years back?

It was them. https://openai.com/research/openai-five

They've since abandoned all their reinforcement learning projects. (aside from RLHF for training ChatGPT)

Did they make a public announcement to that effect? Or do you have insider knowledge?

That was the same time they depreciated OpenAI Gym: https://github.com/openai/gym