Proving things to the IRS is hard.
Let's go back to the stone age--the 1980s.
IRS to my mother: You didn't report your income from XXXX on your tax return.
Mother to IRS: I've never even heard of XXXX let alone worked for them. I'm retired.
IRS to mother: If you believe the 1099 is wrong talk to the company to get them to fix it.
Mother to IRS: I have been unable to even locate XXXX, please provide me with contact info.
IRS to mother: We are not allowed to give out such information.
I was off at college at that point and don't know how the situation was finally resolved.
When I got a bad 1099 my former employer even sent out a letter explaining the problem (a bogus entry had made it onto all of our 1099s) and how to dispute it with the IRS but in that case they weren't going to be contesting it--the company had already gone under. I'm pretty sure my mother's case was an illegal with a forged SS card who used my mother's information--the company would contest a dispute over a 1099 because by their records it was correct.