@NathanHuisman Welcome to the forum!
Yes, FIDO2 PINs can be up to 63 characters long, so it would in principle be possible to use a passphrase consisting of up to 8 words (with word separator characters) or up to 9 words (without separators).
However, I suspect that someone who wanted to reduce the attacker’s probability of success to less than one in a million would rather type a 4-character alphanumeric code (e.g., 8zbU) than a two-word passphrase (e.g., fretful-illusive), especially if they use their passkeys frequently.
On the other hand, for someone who felt it was important to reduce the attacker’s probability of success to at most P ≈10-11, I could perhaps see that a three-word passphrase (e.g., slideshow-yoyo-open) may be more attractive as a PIN than something like a 7-character alphanumeric code (e.g., fP9eLo3)