Snipped for brevity.

This is mostly because pizza basically is a form of bread, just bread with something other than dough baked right with it. I know southern Germany has Flammkuchen, which are basically thin crusted pizza usually topped with creme frèche, bacon and onions and historically baked to check the temperature of the oven before you shove in loaves of bread.

Which is an important thing to check, because otherwise you might burn or under cook your bread, at a time when temperature measuring instruments were extremely expensive if they existed at all, and may not have been able to handle an oven.

I would not be one bit surprised if products like pizza were common across Europe at one point or another so the baker could check the temperature of the oven.

And have a snack.