IMHO for a fast TT the frame needs a serious CFD design process plus wind tunnel testing. Mixing different existing designs in one frame doesn't automatically do the job.
Plus a fast UCI legal TT biken nowadays is aerodinamically designed for much higher speeds than triathlon TTs are, making them fast for their specific purpose, but not for the other (be it TT or tiathlon). Thats why Team Tudor who has BMC as bike sponsor will ride for the 2026 season an old TT model from BMC that was designed for road race TT speeds only, instead of the new BMC TT that is offered as an universal bike for both road race TT and triathlon. And reason is not that BMC is not willing to provide the team with the newest stuff.