IMO the resident was a little inappropriate if this was a *written* eval. Working with someone one shift is not usually sufficient to form a concrete judgement regarding their strengths, unless something really bad happened, which it sounds like wasn't the case. If this was a verbal eval, no harm done. He may just be one of those people who says whatever they're thinking whether it's backed up or not.
The other thing you do to help prove you *are* suited to EM is having thick skin, letting that little comment roll of your back, and working hard to be a good student during the rest of your rotation.