I never understood it that way. It was just that Eichmann and many other prominent monsters of the Nazi era were polite, cordial and unassuming people when you got them alone. That you could never tell what hideous souls they had because they didn't put it on display. They weren't cackling madmen or vicious savages in their personal lives.
I think that interpretation conflates civility politics and banality of evil.
One axis about decorum and appearance. Those who look and act like "good and decent upper middle class people" always get the benefit of the doubt where others do not, even when that appearance, like with Eichmann, completely falls apart on closer inspection.
The other axis distinguishes between people who have a highly ideological agenda and execute on it (evil) and those who are mostly unthinking cogs in a machine of destruction (banal evil).
