This argument is getting insane. You're whipping your cocks out about the technical knowhow of a 15-year old game and nitpicking about when exactly one standard got dropped in favor of another. What's worse, no one here has actually provided any proof, only anecdotal evidence.
So I thought I'd at least try to post some facts, and looked around.
First I tried finding lists of games that used 3Dfx Glide. Guess what game(s)
don't show up? Thief.
Then I looked up Thief specifically in relations to 3Dfx. One search result took me to TLLG, which are the de facto experts on all things Thief. One poster states flat out:
Thief has no Glide support, it's Direct3D only.
However, this is in a thread where a user is trying to get Thief 2 running on a Voodoo 3 card, and it turns out that's the card that Thief 2 looks the best on, hence the effort of the OP there to get a Voodoo 3 card running in 2006. Keep in mind though, that we're talking "native" Thief 2, not fan-patched to oblivion or using NewDark. The Voodoo cards could render stuff like the fog better than other cards, both back when Thief 2 was new and in the years afterwards.
EDIT: Almost forgot:
A list of games that use 3Dfx Glide.
Thief in regards to 3Dfx.