FrogReaver
As long as i get to be the frog
I feel kinda funny about this, and @lowkey13 can correct me if I'm wrong, here, but I suspect the meaning is that 5E doesn't have sharp-enough boundaries between the classes; there isn't enough specific to one class that the other classes cannot acquire; that making one character choice does not do enough to foreclose others; that there are not enough restrictions on the characters.
That to me would be a particular kind of class based system - one with no overlap between what characters of different classes can do well. There's probably a good name for that kind of class based system, but I'm not sure what it would be.