They don't have an N number so it is not registered as an aircraft. It's a toy
I'd say that's about the long and the short of it, for now. I'd note that ultralights are formally called "Ultralight Vehicles" (my emphasis), not aircraft, by 14 CFR 103.
That said, I've read some of the proposed changes to address drones, and...well, the FAA (like all .gov agencies) tends not to regulate lightly. Expect definitional changes, soon.