I don't think it was the LLM solving a Millennium problem—it was the LLM solving a Millennium problem followed immediately by a mathematician claiming that their work had been ripped off.
It's also notable that the Open AI proof may not even be interesting to mathematicians.
Even if people already had an idea that LLMs were training on user inputs, it's the first time it's actually caused an issue. Mathematicians, and plenty of researchers, working in ambitious or competitive field now have a very good reason to avoid LLMs.