The guy sounds like a jerk, but honestly I don't know how I would feel about a tandem acting that way on a group ride.

It sounds kind of annoying to have to get around a defensive riding tandem on a group ride sprint. Why would a tandem even be contesting a sprint? Shouldn't you just get out of the way? I don't know, I've only ever ridden with tandems on brevets.

Then again, if you were holding your line I can't imagine where the difficulty comes from.