The problem with that article is that it seems to rely on a 1979 Revenue Ruling, and the 1040-NR instructions indicate that there was a change in law in 1986 (probably the Tax Reform Act of that year) that changed everything.  I find it odd that articles written since 1986 that refer to RR 79-388 do not mention any change from 1986.  You might contact the author of the article you cite, and ask what happened.