They(F6) are a new set of characters kaido has under him who are ex captains,so some of them may really be calamity level.
Luffy who everybody thinks is above calamity would have been made a headliner if he was willing to join the beasts pirates,just like the F6.
So I see no reason for a cap to be placed on these new ex captains to put them forcfully below all calamities.
Same reason I don't see why King must be forcefully below luffy like many say,even though we know nothing from King.