[SLWW]

Given recent moves to allow Ukrainians everywhere to bear arms in response to the invasion. It's rather reasonable to see that the Civilians, once they enter into defending a stronghold (in this case, just a power plant), that they are now considered a militia, which can surrender.

[thefounder]

But not all civilians bear arms so not all are considered militia thus they can't "surrender"

[Bjartr]

I looked up surrender in a few dictionaries, and none of them even mention this constraint as a secondary definition, much less the primary meaning. It just means: to give oneself up, submit, yield; relinquish possession or control over

So yes, those in charge of the plant surrendered it to the attackers.