A non-resident alien writes (remotely) a paid-for article for a US based publication. Is that US source income? I think not as per:
https://www.irs.gov/Individuals/International-Taxpayers/Nonresident-Aliens---Source-of-Income
in which it says that for salaries, wages and other compensation, the determining factor is the place where services were performed. So if the article was written outside the US, it is not US source income, correct?