Speaking of his native tongue--based on my own experiences working with international colleagues, I think there's also a little bit of a cultural component to his behavior.
This obviously doesn't hold true for all individuals, but Americans tend to prefer the "Let's all hold hands and sing kumbaya" type of social dynamic. "If you don't have something nice to say, don't say anything." Much emphasis is placed on not hurting anyone's feelings, and maintaining the appearance that everyone likes everyone else.
Northern Europeans, meanwhile, tend to prefer the "We're all adults here, so let's cut the bullshit and say exactly what we think." Being honest and genuine is more important than being liked.
This part, I find really significant:
And hey, I don't actually think we've personally even had any problems. And I realize that you may react very strongly and get nervous about us having problems, but realistically, do you actually expect to like all the other kernel engineers? And equally importantly, not everybody has to like you, or necessarily think they have to be liked by you. OK?
That strikes me as a very Northern European view, that is not as likely to be shared by most Americans. It's just not how we're socialized in America. And too often, "professionalism" gets defined as "American style," while anything un-American is deemed unprofessional, which is not quite right.