He was drafted, yes, but it’s not like there were no defecting fascists or other such factions. If he was Jewish (idk Zelensky’s specific ancestry and Judaism is traditionally matrilineal), then he probably sided with the Soviets because the Soviets sought to protect racial minorities from fascist butchers.

Even the “left” opposition of the Makhnovists had a real problem with members murdering Jews and other minorities. The “left” opposition otherwise (like whatever Sholem was doing before he left) had a very small presence.