It's quiet enough that those guys are not completely crazy for not having ear protection — but of course the microphone is not picking up the full volume of the shots either, any more than it does for unsuppressed, supersonic rounds.

Not when you're standing right next to it with no ear protection. The peak is still going to be on the order of 115-125 dB at the muzzle even with the best can. This is well within hearing-safe territory (<140 dB per OHSA). But that is still louder than an auto horn!

As others have noted, recording equipment doesn't register these levels reliably, so watching a recording does not reflect the actual experience.