Steelcore that's true of anything. However I can not see how it could be declared to be a MG if (a) the parts were properly demilled; and (b) you satisfy the "requirements" that are spelled out - not open bolt, etc. etc. In the case of the PKM, you have to rebuild the firing ping such that the bolt can't fire from open bolt, and of course the sear block.
I simply can't see where ATF would take exception because the design changes required to make it semi-only mean that you are literally building a belt-fed AK or VZ-58.