No, the ability specifies what the duration is.
The stat block specifies:
Each of these effects has a spell save DC of 15 and a duration of 1 minute.
The specific duration of "1 minute" overrides and replaces the duration of "concentration""Concentration, up to..." that these spells normally have.
Is this overpowered?
No, not really. Monsters often have one-shot abilities that would likely be broken in the players' hands.
Anyway, there's a lot of ways for this to fail. Consider how much has to go exactly right: First he has to hit with the bolt (and he only gets one try); then the targets get to save against the effect, and then anyone who fails gets to save again each turn (standard for hold person). Are the players going to lose several actions? Yeah, possibly, but it's unlikely they'll be so badly disabled that it makes the whole fight go sideways.