The variant classification is an issue if there is a named restricted or prohibited firearm. The STG44 isn't a named prohib. It is prohibited because one would be either 12-2 or 12-3.
The only applicable issue would be ease of auto conversion, within the Hasselwander standard.
Of course, there is also the possibility that the SFSS just doesn't want to approve it for importation, and is just stalling the process. Easiest way to deal with a situation - just do nothing. After several years, maybe the thing will just go away.
Incidentally there is some garbage in this thread, unrelated to this rifle. The posters can clean up their trash, and save it from being dumped.