That's why I said it was at best "something you know by-proxy". It isn't "something you know" that you share with either the IdP or the RP, but "something you k

That's why I said it was at best "something you know by-proxy". It isn't "something you know" that you share with either the IdP or the RP, but "something you know" that you share with "something you have" (your password manager), which in this case is a proxy known to neither your IdP nor your RP in most scenarios. It's a useful workaround to have "something you know by-proxy", but it doesn't really meet the strict definition of the factor.