Well if someone is a future Nobel laureate, then they are the exception not the rule and they'll probably have no trouble getting into a physics PhD program regardless of their major. It's just very unlikely that any of the posters here are that person... just from a statistics point of view. Most people could not do well in physics graduate courses or physics research without understanding basic physics. However one gains this knowledge is pretty much irrelevant, except that it helps admissions committees to see a transcript with a standard curriculum and good grades and to get letters of recommendation from physics professors. If someone can do a good enough job of showing an admissions committee that they are qualified to study graduate physics without any physics classes, then I'm sure they'll be fine. However, the tiny percentage of people who are actually this smart and this qualified are not the people that come to this forum asking whether taking the physics GRE will get them into a physics PhD program. The people asking this question are the other 99.9%, and the easiest way for them to be prepared and to show that they are prepared is probably to take some classes.
"Do you think non-physics undergraduates (but whose undergraduate research has strong elements of physics) are really less suited to graduate study in physics than typical physics majors?"
In the vast majority of cases, yes. I don't mean to say that you have to take classes to learn physics. This would be preposterous. I'm just saying a good amount of knowledge is necessary to succeed in graduate school and beyond. You need to have it, and you need a way to show you have it, along with discipline, work ethic, and a passion for physics.