The sophistication of these guys is high. They're hiring US citizens to interview for them and then if they get hired, their work quality is high so they fly under the radar for awhile.
I've thought about the same thing. My company specializes in blocking candidate fraud and we have yet to see anyone who's sentiment isn't "get these people out of here".
Employing a North Korean can create sanctions and criminal risk, so it's not worth it.
From what I've heard from people who have accidentally hired them though, many are great engineers.