The letter makes no arguments why AI companies proving a bunch of results quickly at all affects the ability of mathematicians to do what they want to do. It's just a giant self-congratulatory list of all the wonderful things the mathematical community does and notes that AI companies aren't doing the same thing.
It's not enough for them that they are still free to pursue all the goals they outline in that letter to their hearts content. They want to make sure everyone else doing mathematics also sets aside their own goals and pursue theirs. I distrust AI as a technology regardless of alignment. And I've learned to just not use them and focus on the things in my life that matter to me. It turns out that I'm still free to pursue the things in my life that are meaningful regardless of the existence of LLMs.