On one side, yes we don't have hard evidence that intentional plagiarism is exactly what happened.

On the other side, the lack of evidence is pretty damning. Only OpenAI can try to prove that they came by these results legitimately, and the case they're making is quite weak. They could make public metadata about what their model was trained on and whether it did train on the conversations in question; they have not. TBQH I read it as even they don't know.

And regardless of whether the result is legitimately obtained by their model, they've not at all conducted themselves well throughout this story. They set out to scoop researchers based on a rumor. They threatened to ruin a mathematicians career. They put up a paper that deliberately doesn't cite the most relevant research, despite building directly on it. No matter how you look at it, OpenAI has and should lose any standing they had in the research community.