Earlier quoted context omitted.
It's a specific ideology that has ties to white supremacists and other racists.
You can tie almost anything to anything if you squint hard enough, and I'm immediately on my qui vive when someone tries to make such a claim. I'd have to read into it in more detail to be sure. In the mean time, apologies for initial confusion, and consider me somewhat warned.
The letter links to an exposé on the racist ideology behind longtermism. You don't have to take my word for it.