Although it sometimes does investigations outside of this, most of the information the IRS garners is from information ‘voluntarily’ reported (but required by law). For some reason CX has chosen to do this for non-US nationals not residing in the US — regardless of whether or not they actually incur a real tax liability in the US. So no matter what you are on their radar where you otherwise might not have been.
Many people don’t file at all. Now the bad news is that if you have tax liability the IRS eventually catches up with you and can tack on a whole bunch of penalties and interest (there is a whole industry of lawyers in the US to help people get out of this hole — usually caused by not filing and the overwhelming majority of money owed being penalties and interest which the lawyers mitigate for a quick payment deal). They also can just freeze and take assets in the US — bank accounts, property, etc.
So I guess the real impact comes from one’s exposure to assets held in the US or under US control. If it’s none, this is more of an annoyance. If it’s a lot I’d suggest they get competent tax advice pronto. US tax law is one of the most complex and conflicting things on the planet and even tax experts often disagree. Quantum physics is easy and more straightforward by comparison.
As usual, CX has decided simply to ‘1099’ people and dump it on to the individual to sort out.