Fruit is a botanical term for things with seeds in it. Vegetable is a culinary term. They aren't mutually exclusive so when people say something is either a fruit OR a vegetable, it isn't an apples vs oranges comparison.
I mean, that it's a terrible definition of vegetable, considering it rules out a very large amount of vegetables, including tomatoes, bell peppers, eggplants and cucumbers.
You can apply that to pretty much any categorization we make for the natural world.
The natural world doesn't care for our categorizations or social construct. It can, and will, make shit that just does not fit to any of our boxes, and like it!
It's a definition of "fruit": thing that's full of seeds, intended to fall off the plant.
Technically, any part of a plant is a "vegetable". Tomatoes? Vegetables. Lettuce leaves? Vegetables. Leek stalks? Vegetables. Spruce 2x4? Vegetables...
Why should anyone care this much though. It isn’t poisoning anyone. it’s not breaking anything in the kitchen. It’s still edible. No animal died to give it to you.just Eat it and be fed.
Generally when people talk fruits and vegetables they are referring to culinary tradition. Vegetable does not have a precise scientific definition anyway, despite what you've heard from The Big Bang Theory.
Most pizza is white bread, marinara sauce, cheese, and less healthy toppings like red or cured meat. Basically pizza is carbs and fat, minimal micro nutrients, and moderate protein. The amount most people need to eat to feel sated is generally more calories than a meal centered around beans, lean meats, or other proteins.
There is also a lot of variability. Deep dish pizzas or some pan pizzas from some chains are soaked in oil in the pan when cooked (looking at you pizza hut). Certain sauces also boost the calorie count, such as ranch or Alfredo.
Likewise, a thin crust pizza with lots of veggies is comparatively ok in terms of calories per slice.
I always mentally treat pizza the same a pasta. It's a potential to carb and calorie bomb yourself without feeling sated for long, but can be prepared in a way to be better and part of a rounded meal. Eating a couple slices with a salad is fine. Eating a whole pizza on to of other regular meals in the day isn't.