They didn't "fork" it, but rather implemented the ideas in it, that's science and engineering as intended. For open source software projects on the other hand, the intended behavior is contributing directly, and forking is a distant plan B.

The product OpenAI got mega traction with was based on the GPT series of papers, all of which were published by OpenAI and all of which are major advancements in research.

Also OpenAI was founded before the transformer paper and spent most of their early years focused on RL style research and products - just look at what they published pre 2019.

Implemented a paper, yes. But without a dataset, its pretty much worthless.