OpenAI always had an "official" framework - it just used to be Tensorflow. That's why many of its public packages (baselines, spinningup, blocksparse, etc.) were built with Tensorflow.

OpenAI has researchers, and it has people who work on infrastructure for the researchers. Researchers are free to use whatever they want, but if the infrastructure developers want to build something for the researchers, it's beneficial to have a "standard".

OpenAI researchers will still be able to use other frameworks for their own research. All this means is that their major infrastructural projects will be released in PyTorch.