I think that part is to test soft skills, being able to work under stress and talk to a person is a part of being a good team mate. Straining both at the same time is a feature, since it is much easier to fake soft skills when you aren't distracted by a technical problem.

It is much more stressful than a real work situation, true, but people work much harder to appear nice and helpful in an interview setting as well so having a harsher test than real world situations to test soft skills seems appropriate.

There was a study conducted some time ago within the past year or so (it had a long discussion thread here, naturally) where it was discovered that when allowed to solve a whiteboard puzzle in a room alone, candidates performed far better. Now obviously communications soft skills must be tested. But perhaps the format can be tweaked so the candidate has some time to crack at a problem on their own prior to be asked to explain it.

The "niceness" of the interviewers is irrelevant. The power differential at stake sparks a survival instinct that leads to stress. (e.g. they literally hold power over your future meal prospects.) Though perhaps in others, it invigorates them with a sense of purpose, cool, and collectedness. Perhaps that is truly the 10x engineer.