I would say the Sithi ARE elves, just a different take on them. Like with many classical Fantasy Tropes, Tad subverts it slightly/invents his own version of them.
Being this true, I prefer not to say it, though. The Sithi are Sithi, that’s how the author wrote them, and despite in so many things they remind us of the classic Elves, they aren’t in other ways, like the open-minded way of talking, the provocative dressing ways - which are not even intentionally provocative per se, but it’s rather a human perception - and the way they socially interact, more open and carefree than the actual Elves. Not to mention their names, their physical appearances...
I don’t know, I am an author myself and I’d not like to read that some of my characters or creations are just “another take of/ a different take of *insert your common race/trope here*”, even if it’s true or, as you well said, it’s a subversion or a reimagining. Because that subversion/reimagining is valid and has right to exist in its own. I ignore Tad Williams’ position on the Sithi, though, but definitely they are not Elves for me and I would not say they are. My personal preference, of course. It’s still interesting to debate about the creative process, as you did.
It’s like saying the Dothraki of G.R.R. Martin and the Thritings - is that how their name is written in English? - of Tad Williams are also the same thing. Obviously “inspired”, “similar”? Yes. But not the same thing.
Btw, this is my first ask in this new side blog of me, thank you so much for sharing your thoughts. :)