Their manual description *is* pretty excessive when looked at next to the other races... but aside from that they're hardly "Mary Sues." They've apparently done exactly zilch beyond "sit back and watch" in the known history of the world, you never see them except as PCs and shopkeepers, etc... plus, they're 8000 year old weird mystical beings that die instantly to a random stone block 3 steps into a dungeon, or because they accidentally put on the wrong piece of jewelry and couldn't take it off within the 10 seconds it would take to poison them to death. To be Mary Sues for real they'd have to be this newly-introduced species that's suddenly the focus of the whole game/world, but instead they're the rare oddball that's hard to play if you don't know what you're doing (avoid traps, don't kick doors, etc...) and have almost no non-player presence in the game at all.
I was a bit "huh? MORE elves?" about a 4th type of elves when I first saw the two new races mentioned, but after seeing what they're actually like, they really are "different enough" for me to be okay with them. If anything, High Elves and Gray Elves are the ones that don't really stand out (aside from "these are the tough Legolas elves and these are the squishy magic elves" which... compared to Dark Elves' crazy spider religion and Mist Elves' "we're weird pale fragile 8000-year-old things formed from magical mist stuff with a metal allergy" thing, that's not really much as far as distinguishing features go. I suppose having two types of "generic elf" isn't too bad, though. It's not like there's those two plus Wood Elves and Mountain Elves and Hill Elves and Sewer Elves and... you get the idea.)
...Have the graphical-mode sprites of the Mist Elves ever been updated to actually look like what their descriptions in-game sound like, by the way? Like, white skin white hair white eyes... not just the "basic blonde-haired elf" look that I saw in screenshots a while back. The white-everything descriptions they have made me think they'd look a lot paler and less-normal-elf-ish in graphics... from all the descriptions I was sort of picturing them as these weird wispy glowy pale almost elemental-ish things that just happen to be elf-shaped, rather than something so close to regular elves.