You're right, the people who are generally attracted to Bogleheads in the first place, and furthermore the people who are able to participate there without getting removed, are probably quite civil under their FB/Twitter/Reddit handles too. Or they're people who won't go anywhere near FB/Twitter/Reddit.
What's obviously very hard is allowing discussion of politics, macroeconomics, religion, race, etc. without it getting heated. Bogleheads doesn't even try.