I live in Seattle, and this is 100% correct. I even had one tell me that TAMU wasn't even the first to use it, then proceeded to tell me a story about a guy that came out of the stands, and stood on the sidelines in uniform waiting to play, and that was where the 12th Man came from. But it most definitely wasn't at TAMU.
I was like, "uh..."
And yes, they refer to themselves as "12s", the official slogan is "We are 12", and I think their highest selling jersey is the "12" jersey because, as one "fan" told me, "it's cheaper, and I don't have to waste money on a guy who's not going to be here that long anyway".
tl;dr Seahawk fans are the WORST.