The suppressor/gun does not know if it is inside a room or in an open field. It produces the same amount of sound energy no matter where it is located. I agree that you can get a variety of measured energy readings depending on where you place the microphone/meter and that is why there are standards for testing. That said, the change in readings is not due to the suppressor/gun producing more energy just because it is in an enclosure. The change in meter readings must be due to other factors.
You'll pick up higher dBs on the mic in an enclosed area for the same reason your ears do. No, the SPL at muzzle is no different, but we don't measure right at muzzle, nor do we put our ears there unless one is a complete moron. What is higher is the SPL at any distance from the muzzle when there is less space for the pressure to attenuate.
Your contention was
the peak sound intensity (what is measured in suppressor performance) is NOT higher in an enclosed room as that would violate the laws of physics,
The peak intensity as we measure it, 1 meter left of muzzle, 1.6m off the ground, with microphone vertical, does attenuate from what it is right at muzzle. How much it attenuates depends on environmental factors, and that includes walls & ceilings that constrain. There are reasons for MIL-STD-1474D:
5.4.5 Transducer locations
For shoulder-fired and hand-held weapons, transducers
shall be located at the center of each operator or
crewmember's probable head location. For other weapons
the transducer shall be positioned 1.60 m above the
ground surface; for sitting locations it shall be 80 cm above
the seat. When the operator must be present, the
measurement shall be made 15 cm from the ear closest to
the noise source (i.e., muzzle or breech, as the case may
be) on a line between the operator's ear and the noise