I don’t think that’s right. Why would they call it “tier” then? The whole point of tier is for the bonus.
I think that line implies that the current 5 slots of tier, similar to the domination set pieces, require at least one of, to get the tier bonuses.
You can’t get the bonus from 2 or 4 pieces of the other slots without at least 1 tier piece coming from the existing 5.
Otherwise the whole labeling of “tier pieces” would be a huge troll on players - especially when they’re putting in a week cooldown.