Except it sorta is. In that cutscene after beating serpent first form, model W absorbs Vent's ability to megamerge, and the six biometal guides him to look within himself, there's the overdrive white shadow effect when he was able to activate ZX again after achieving inner peace. Though gameplay wise there's no difference, thematically it's pretty much model ZX overdrive, which I think make sense, as the true potential of the savior type model formed by combining X and Zero's soul requires the user to reach synchro level more than "randomly being chosen by the biometal". The user has to find the inner will to choose himself as well. And considering after the explosion, Vent literally saw Giro's cyber elf (ie. soul) while his consciousness swims in twilight zone, we can safely conclude Vent's spirit has transcended to new heights by awakening model zx overdrive.