All of the major operating systems and all of the major browsers are password managers. Apple, Google, Microsoft, and Mozilla are all password managers. They all have apps that let you access their password managers on other operating systems.

You're right that all of the major password managers like their lock in. The Credential Exchange Protocol is just barely good enough that OS vendors can say they "support" it, but tricky enough to find that ordinary users probably will never try it. (Not to mention that it doesn't even work yet on Windows or Android.)

As for attestation, the good news is that Apple always returns 0s for the attestation ID (because Apple, like you, opposes it as a side channel), and so any public site/app that insists on attestation would reject all Apple devices. This gives smaller password managers like Bitwarden sufficient cover to 0-out their attestation as well.