Is spaghetti and bechamel sauce an authentic Italian dish?
In Italy, besciamella is usually only used on pasta when it's going to be cooked in the oven: pasta al gratin, pasta al forno, or as part of lasagne.

As for your question - is it authentic to just toss some pasta with it? I would use the same method I'd use to answer the infamous "is X a sandwich?" questions: if somebody asked for a plate of spaghetti at a restaurant and they came covered in besciamella, I think basically every Italian would be baffled. I've never seen it on a menu, never seen it on a table at anyone's place and never heard it discussed until I read this discussion, which makes me inclined to say it's not authentic. It could still be delicious though!