That's why I mentioned that it is for a setting book to expand on it. They have the stats of the subrace and a short blurb about them I really don't need any more than that for a UA article, they could have even gone "This is how we'd do sea elves" and left it at that. If they later add them to a book then that is where I'd like to see more information in how they fit into a setting but in an article providing playtest material, not so much.
But wouldn't the culture depend on the DMs world?
Possibly, but there are likely to be some common reasoning as to why this elven subrace exists and what makes it distinct enough from the other elves to count as a new one. (This can be pretty obvious, like Sea elves, or rather obscure, like Wild elves, Gray elves, Moon elves etc.)
Even in Eberron, where there are only three elven subraces: Elves, Drow, and Sea elves, there are commonalities to other settings. Between FR and Greyhawk, or a more generic fantasy homebrew, the places of the elven subraces are much more similar.
