[2020] FFP ranked - my mates and me (Diamond II and 2x Platin III) got a Bronze IV to fill up our squad lobby today. But we can’t queue with our mates that are Silver IV, because that would disrupt the integrity of matchmaking.

You can't form a premade team with a player who is greatly different in rank because that is something that is in your control - and thus it is repeatable and abusable.

As a lower ranked player, you cannot queue up with randoms and consistently be paired with diamond players to carry you and boost your rank, which makes this not abusable.