This seems like an accurate claim to me. I do not put butter on my sandwiches, nor do I know anybody who does that, nor can I think of any restaurant that serves sandwiches that way. Source: I’ve been American my whole life.
I do butter my toast though, I may need to give this a try. Would I substitute the mayonnaise for butter? I just recently discovered grilled cheese sandwiches fried in mayo instead of butter.