I think, in fact, "not being around other people makes lots of healthy, ordinary people feel depressed and anxious" is just about as robust a finding in human psychology as you're likely to find.

Agreed. But if the only people you are around and depend on for your mental health is your coworkers, that is unhealthy. There should be other relationships in your life that are meeting your social needs outside of work. If that is not the case, it is unhealthy.