That would be a pretty pointless conversion, too much effort for zero payoff in this scenario.
Foreign training would explain use of an AK74 though.
I wasn’t saying that is what it was, just saying that the case (even if they got it right that it was 5.45 x 39mm) is no absolute proof that it came from an AK-74.
And there are some ballistic advantages to the 5.45 x 39mm even in an AR platform, especially if you think you might be encountering people wearing body armor and can get ahold of the ‘good stuff ammo’ .
And it is hardly a high effort conversion. You can buy the complete weapon, or swapping uppers is just a matter of pushing two pins out, then slap the new upper in, and push the pins back.
Not that this is an argument that is worth getting into at present.