It's like there are two categories of PIZZA FOOD: pizza FLAVOR stuff, which is where you find french bread pizza and tortilla pizza and english muffin pizza and the rest - what makes them pizza is the flavor combo of tomato/cheese/whatever. Then there are pizza APPEARANCE things, which is where the cheesecake pizza and cookie-cakes and whatever exist. And then there's what we talk about on PM, which is what counts
Goodness knows, we're the only ones that care about this stuff
On a side note, speaking of french bread pizza, for me the difference is: was the base already baked before toppings added or not? If the toppings were added on the french bread dough and then it was baked, it's pizza. If topping were added after the bread was baked, it's an open-face sandwich.