Yeah, I just don't agree.
I looked through all of the cards that don't really make sense for FR (or really,
only FR).
- Mordenkainen
- Orb of Dragonkind
- Acererak the Archlich
- Eye of Vecna
- Hand of Vecna
- Book of Vile Darkness
Mordenkainen and Acererak have both visited Forgotten Realms, but neither use it as their main home even in 5e (Mord has a tower in Avernus, and Acererak leaves FR after being foiled in Tomb of Annihilation). The Orbs of Dragonkind have also been on FR at times, but the Forgotten Realms wiki acknowledges they were probably formed on Krynn. The Book of Vile Darkness has some copies in FR, but the original was penned on Greyhawk. The Eye and Hand of Vecna have never been in FR that I have seen, and Vecna has no worshippers there.
It seems that by your definition, any character or item that has been on FR once suddenly becomes primarily FR property, which I don't hold to. Even if that was somehow true, it still doesn't apply to the Eye or Hand of Vecna which are present despite not having any relation to Toril at all.
So no, I don't think WotC actually considers these 6 characters/items "FR IP," but I do think they are all just too iconic to D&D for the team to skip on when making a D&D card set. So they added a few cards that aren't really FR at all because if we really stuck to strict setting lines, Mord and Acererak would probably never get cards because Greyhawk is not going to get its own cardset ever.