Just failed this side quest the other day, and pretty sure I recall it saying "As an action, release a prisoner" (limit one at a time)
I'm pretty certain it doesn't say that. We did look and played it twice without seeing such text. It's not impossible we had collective blindness but this post makes no mention of an action either
We played it as an action which was much harder than most of the other quests (there was also a point where we couldn't get past a monster in a critical spot)
I'm definitely leaning towards prisoner release not costing an action and the "one at a time" text purely being there to let the app track how many you release without them having to program the extra options.
EDIT: It's also worth noting that you don't have to do the whole map (and probably aren't expected to) in order to "win" this one.