Bigger Perfidus would be nice, sure.
But his whole discussion seems a little off.
First, afaik, Perfidus do not exist in the original Ravenloft setting. It is a creation for the server that fills a role needed by the server. As it was pointed above: it makes no sense creating a domain/dungeon/area that is not playable.
Perfidus is not a trivial area/dungeon (between open path and competent group). Gathering adamantine has its own problems. If gathering adamantine was so easy, the market would be thriving and it is not.
In the end, the only conflict that exists is a metagaming conflict. Some players know that a pit fiend is an endboss in DnD, but this is not the case in the domain Perfidus, a tailored domain in a persistent world for a videogame launched twenty years ago.
In game design, Perfidus is Perfect. It is a marvelous combination of luck, skill and power. And the reward is good.
Lorewise, I do agree that chosing another monster to roam the place would be more akin to DnD 3.5. But is that where the developers should spend their time? Re-skining a monster?
In many ocasions the disbeliefe was suspended: gold has no weight, you can palm via SoH 99 darts or shurikens, but cannot one key, skills, spells, feats and classes were changed. If warmage was deeply changed in its mechanics, why not Pit Fiends?