ByteDance has been working on autoregressive image generation for a while (see VAR, NeurIPS 2024 best paper). Traditionally they weren't in the open-source gang though.
The VAR paper is very impressive. I wonder if OpenAI did something similar. But the main contribution in the new GPT-4o feature doesn't seem to be just image quality (which VAR seems to focus on), but also massively enhanced prompt understanding.
They haven't been focusing attention on images because the most used image models have been open source. Now they might have a target to beat.