>>96387120
It's more like there was more than one type of "veteran sergeants". Actual vet sarges and vet sarges promoted to the first rank of centurions but still NCOs without their own commnand.

In legions like the SoH, which was leaving behind the imperial ranks, this encouraged these guys to get their own warband like that dude at Tizca.
But it happened also in other legions. Remember that Aeonid Thiel did the same, he was a vet sergeant that ended up leading their own Red Marked vet squads detachment.