This room is like a One Room Hold. To complete it, you need to pass through it several times.
Specifically: (SPOILERS! You don't need to read this secret stuff, I just included it to be sure we're talking about the same thing.)
Click here to view the secret text
×1 - Enter the room and reach the stairs.
2 - From the level entrance accessed by the stairs, conquer the room, and reach the stairs again.
3 - From the level entrance accessed by the stairs, exit the room.
Assuming this is your first pass through the room, you need to reconsider your second assumption. (This secret is your hint.)
Click here to view the secret text
×What else, besides a mimic, can trigger that pressure plate?
Good luck!