If it was just skill in choosing the correct items, you can implement that with just giving a large pool of items for everyone to choose from to keep things fair.
But considering Rex is entering with two cheat items (and likely going to win because of them) which he only got from a high-end dungeon normally only beatable by high-level players, the idea that people are able to enter with items only obtainable at levels much higher than 25 means it's still not really fair to people who are legitimately level 25. Given, that specific boss he killed had a cheese strat that would have let lower level parties clear it too, but that's probably not true for the majority of high-level bosses who're dropping high-level loot people would be able to use in this.
He's basically being a level 19 twink in warsong gulch right now (is that still a thing that people do?), wearing gear he's technically allowed to but not supposed to actually have access to at that point.