If all they really have is some API requests, even assuming they're real and originated by Deepseek, that's very far from proof that any of it was used as training data. And honestly, short of commiting crimes against Deepseek(hacking), I'm not sure how they even could prove that at this point, from their side alone.
And what's even more certain is that a vague insistence that evidence exists, accompanied by a denial to shed any more light on the specifics, is about as informative as saying nothing at all. It's not like OpenAI and Microsoft have a habit of transparency and honesty in their communication with the public, as proven by an endless laundry list of dishonest and subversive behaviour.
In conclusion, I don't see why I should give this any more credence than I would a random anon on 4chan claiming a pizza place in Washington DC is the centre of a child sex trafficking ring.
P.S: And to be clear, I really don't care if it is true. If anything, I hope it is; it would be karmic justice at its finest.