Portals are interdimensional gateways. The players have freekin' interdimensional squids in their heads.
I don’t see how that’s relevant. Full grown mind flayers can Plane Shift just themselves once per day, not teleport all over the place at will. If your point is that *shrug* it’s fantasy and anything is possible, I would point out that world building and magic without rules and defined limitations leads to weak storytelling and unsatisfying gameplay.
Let’s just recognise that the portals are just a legacy of the waypoint shrines from the DOS games and there are several ways they could be better adapted to 5E and Faerun.
