Hmm, that is indeed curious about the disc tab. The Zeta TL was definitely marketed as the "improved" successor to the Mozo Pro, so maybe as you say it's intended to be lighter. Even dream bikes in those days used XTR V-brakes (I thought discs were a gimmick - imagine!).
The 2000 model Zeta TL (the year after mine) did have a disc tab though.
Crud is indeed right about the Zeta's cast magnesium one-piece lowers and brace v the bolted on the Mozo. I think the one-piece is an obvious improvement though - after all, how many times do you ever need to replace a brace? And all the best forks nowadays have the one-piece.