Well exactly! As you correctly point out, by their logic, most quiches or calzones would become another food type as soon as they’re served or a bite is taken.

I don’t agree with their theory, personally, but it can be helpful to understand their reasoning even if it has an extreme structuralist bent that makes it rather distasteful.

It is referring to only one slice of pumpkin pie and not a whole pie. A whole pumpkin pie would also be a quiche. Likewise, a single slice of key lime pie would be toast.