(nods)
It's sometimes helpful to draw a distinction between "lots of people do X" and "nobody is allowed to do Y."
The groupthink Alsadius is positing is the latter; it involves nobody being allowed to express certain sentiments. As I said, I don't see where he's getting his confidence that this is true, as I don't see much compelling evidence for it, but accepting it as a hypothetical I agree that the "safety valve" theory he's talking about follows from it.
The groupthink you're positing is the former and suggests different tactics.
FWIW, I don't think it is true - you don't have far to go to find a claim that, say, women are crazy, or black people steal, or half a dozen other terribly politically incorrect things(true ones and false ones). But a big part of the reason is because we have these unofficial lines of communication. Good luck finding official data on things like racial crime stats - self-censorship has basically destroyed that. Chris Rock is all we're left with.
As Multiheaded added, "Personal is Political" stuff like gender relations, etc also may belong here.