it’s absolutely possible to reduce the number of tries to hits to a manageable level for a distributed botnet that can’t easily be blocked.

This is very hard for me to imagine. Even if the people running the website are incredibly incompetent, they would have to be intentionally moronic about passkey generation for what you are envisioning to be realistic. You are right that passkeys enable brute forcing on a scale regular passwords don’t, but it doesn’t matter, because normal passkeys are just too long for this to be a worry.