If the app has verification tools then this might be legit mail. I would like to know if any other users are getting the same mail after they ask the support to unfreeze their account.
This is the answer, and I'm surprised everyone is surprised by the request!
They explicitly said in their request "simple verification using the Poloniex app" they need a visual verification, they will ask for a mugshot or two during these to prove you're an actual person, every single exchange has one of those, and it's easier for them since it's fully automated.
Again, surprised by the amount of users that treat it like some sort of odd request when it's the norm!