I think “not actual pizza” is one that isn’t a flat bread baked with optional items on top. We don’t know where that word was used before Greek was spoken, or what it meant before it meant “pie” - but we can observe how people use it now.
If I give you a steak and I say “this is a pizza”, you’ll say it’s not pizza. If I open a box of crackers, take one out, put little bits of pepperoni on it, and hand it to you saying “this is a pizza”, you might say it’s a pretend pizza.
You’re right that in some way if I bring you spanakopita, or American-style apple pie, and I say “this is a pizza”, there could be some legitimate confusion - but I doubt it. And anyone who wasn’t expecting a lesson in Greek (or wasn’t Greek themselves) would immediately say those are not pizza.
Words in any language might not mean what they originally meant in a different language.