This is insane and Europe is going to the same direction. Back in the 90's the definition was still very simple: two rounds in a magazine, insert magazine, chamber a round, pull the trigger and if both of them are fired with one trigger pull, it's full auto. If not, it isn't, and that was it. Lots of M16 receivers were used to build semi auto (and with appropriate paperwork, full auto) rifles back then - and it's still possible. Same thing with AK 3-hole receiver guns, there were plenty of them around as semi auto rifles. Usually with just the disconnector tang cut off and otherwise unmodified milspec configuration. I think I still have about a dozen XM177, M16A2 and M4 full auto receivers in a parts bin somewhere. A number of "deko" shops sell parts kits that consist of stripped down M16:s sans barrel and bolt. In current anti-gun climate it probably is only a matter of time when they'll be outlawed.
I used to laugh my proverbial behind off in the 80's when I heard that lightning links and drop-in auto sears are considered machine guns in the US. I had a DIAS as a key ring at the time. A marking punch dimple in a strategic spot on a receiver? In normal circumstances I'd say gimme a break, but having seen what ATF is (and isn't, as in common sense) capable of, this doesn't surprise me much.