That isn't how the magic threshold system works.
The trident isn't magical, the sahaugin wielding it is magical. It is so magical that if it did pick up a +2 (or whatever) trident, it wouldn't be of any benefit. If it picked up a +3 (again, or whatever,) it would be sufficient enough to give an additional +1.
It's not a perfect solution, however, the DM doesn't need verisimilitude, the DM needs a set of numbers that will work that don't become broken when the monster is wielding the parcel as they are supposed to. If you'd like to make those parcels line up with the monsters, cool, it just isn't necessary.
Anyway, the MM also lists the standard equipment the monster has. The parcel system just isn't part of the loot.
More importantly -- how does 3E compare any better at all?? "Recommended wealth guidelines" are a bit lacking. And if you give monsters magic items and have them wield them, it throws off the CR much worse than the 4E magic threshold does. And 2E and 1E... heh.