> but it's also no secret that ChatGPT (and Claude) are used to generate a lot of synthetic data to train other models

Is it true? The main part of training any modern model is finetuning, and by sending prompts to your competitors en masse to generate your dataset you're essentially giving up your know-how. Anthropic themselves do it on early snapshots of their own models, I don't see a problem believing DeepSeek when they claim to have trained v3 on early R1's outputs.