Pizza folded in half is still pizza, it's not a sandwich or a calzone. If you put popsicle sticks into a bowl of fettuccine alfredo it's still fettuccine alfredo, it's not popsicles.
I'm not saying it IS one of those things, but they are probably a better description of what it has become than a slice of pizza.Definitions of things have a pretty wide range. Some things match up exactly with what you think of when you think of pizza. Some things less so.
A slice of pizza blended up and poured into a cup would be unlikely to match anyone's mental image of a pizza slice, but technically I guess you can say that it's a slice of pizza.
An open-faced sandwich is more like a salad than a sandwich. By the same token, a slice of pizza folded completely in half is a different (and worse) version of what a slice of pizza should be.