I'm not sure whether it has been said before, but the main reason why I think that model railroaders would object that LEGO trains are model trains is the track gauge. For trains to be true to scale, they would have to be about 10 studs wide, but usually they are somewhere between 6 and 8 studs. It was never really a matter when talking to the public at train shows, I guess most people won't even notice, but at some occassions I was asked what gauge the LEGO track is, or what scale it is - by model railroaders. And then you unwillingly have to admit that there are actually two different scales, one for the track and one for the trains.
Honestly I don't care whether someone wants to call his LEGO trains model trains or not, but if you want to discuss this with a model railroader you will need to find some excuses why your trains aren't true to scale, which probably leads us back to track geometry.