I'm thinking he really did it, or knows enough about guns to know it "could" be done.
The .223 case-head to shoulder length (1.557") is close enough to the 7.62x39 case-head to rifling leade length (1.528") that I believe you could pretty easily pound one in the chamber with the SKS bolt handle.
The crush fit would hold it back against the bolt face so the firing pin would hit it.
There would be very little danger, as there would be no real chamber pressure developed.
The .224 bullet would just rattle down the .310" barrel and all the gas pressure would just leak past it.
Of course, there is little chance the extractor would hook the much smaller .223 case rim to extract it after you beat it in there and fired it.
The end result would be exactly what he said it is.
rc