Split-phase power cannot be called 2-phase because that name has been defined to mean something else. If you want to argue that split-phase power is single phase, then by applying the exact same logic, you're arguing that 6-phase power is incorrectly named. 6-phase power can be derived from 3-phase power exactly the same way split-phase is derived from a single phase, yet it's not called split-3-phase. It's called 6-phase because that's what it is, and the name wasn't taken beforehand to mean something else.
