No, the room doesn't "know" if a sub is sealed. Ported subs have steeper roll off characteristics below their tuning point (24dB/octave), compared to 12dB/octave for sealed enclosure, so with ported subs it isn't as obvious.
Also, room gain is physics, when a room is smaller, the boundaries reflect low frequencies and augmenting them. There is a formula to calculate what frequency room gain occur. It doesn't just happen with one type of subwoofer enclosure. Hell if you play the lowest note on a bass guitar in a small room you'll notice room gain.
Room gain is a room thing related to acoustic size, unrelated to what kind of woofer is or alignment is used.
What happens is that if you think of free space, you can easily picture the sound radiates away from the source. When you put a speaker in a room, the sound also radiates BUT when the frequency is low enough so that the longest acoustic dimension is about a half wavelength or less, then the room acts to confine the pressure instead of it bouncing around and being absorbed.
In an absolutely solid concrete bunker, once your well below the lowest room mode, room pressure is proportional to driver displacement.
From the speaker point of view, this provides 12dB per octave gain as the frequency falls. In a perfect world like this, that slope EXACTLY compensates for a sealed box's roll off which also reaches 12 dB/oct.
That reciprocal set of slopes fix or EQ each other and the response extends flat to DC.
One of the extraordinarily few cases of a nearly free lunch.
In a real room, not a 3 foot think concrete walled airtight bunker, the slope is always less than 12 dB per octave due to flexing and leakage.
Rooms I have measured (and this would be from a number of averages) tended to be in the 3 to 9dB per octave range, open plan houses having the least room gain.
Sealed box speakers have an asymptotic roll off that is 12 dB per octave although in the region of the knee, it can easily be less if it is a low Q alignment.
Vented boxes, passive radiator alignments have an asymptotic roll off of 24 dB per octave, basically, the higher the order of the alignment, the steeper the roll off is.
Tapped horns tend to be an in between alignment, typically in the 15 to 24 dB per octave range depending on the horn size relative to wavelength.
You can hear room gain yourself in your car where the room gain corner F is usually around 50 - 60 Hz. Here, a small sealed box with a 50Hz corner will deliver deep bass in the car when the windows are up.
Hope this helps.
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