While I agree with the sentiment that 5 Passkeys for 2SV might fall short in some cases (not in my particular one right now, but by a little, I have 4).
This advice to switch to synced Passkeys I think is a valid one.
However, it should be pointed that it might not satisfy everyone, as synced Passkeys should be, by default, considered less secure that the device-bound ones.
Device-bound Passkeys can really be considered another authentication factor (something you have). Synced Passkeys can not (strictly speaking).
With the exception of attacks like YSA-2024-03, as I said in the other thread.