OpenAI's future investments -- billions -- were just threatened to be undercut by several orders of magnitude by a competitor. It's in their best interests to cast doubt on that competitor's achievements. If they can do so by implying that OpenAI are in fact the source of most of the DeepSeek's performance then all the better.
It doesn't matter whether there's a compelling legal argument around copyright, or even if it's true that they actually copied. It just needs to be plausible enough that OpenAI can make a reasonable case for continuing investment at the levels it's historically attained.
And plausibility is something they've handily achieved with this announcement -- the sentiment on HN at least is that it is indeed plausible that DeepSeek trained on OpenAI. Which means there's now doubt that a DeepSeek-level model could be trained without making use of OpenAI's substantial levels of investment. Which is the only thing that OpenAI should be caring about.