[dwaite]

I consider possession of my device and a knowledge challenge in the form of the password and pin to be two factors. Use of a biometric in lieu of password is also two factors.

I don't see a way in which having the possession factor be on my keys is stronger than having it be in my laptop. In fact, for sites that require it my U2F key is in my laptop (Yubikey nano-C).

(Aside: That doesn't limit the usefulness of having a possession factor that is portable between devices, just I don't think it is necessarily stronger)

This is actually why I very rarely opt into the 2FA features of websites - I figure I already have two factors protecting me, but not necessarily factors recognized by the site.