The new staircase could be a fire exit, as it's identified as "egress," though it seems odd that a fire escape would lead into a sterile area. One person opens that door and it would be a headache for the entire airport. As a result, I'm guessing it's the main entrance (airside), but who knows.
I can't believe an emergency exit would get you from landside to airside. Like you say, if someone opens that door and exits, that would likely require a massive purge of everyone in at least T3F, and quite possibly T3E and International-G. No way. This lounge is airside.