Earlier quoted context omitted.
He's discussing this in the context of writing code, not typography. I think it's fair to assume we're starting with the premise that we would only choose characters that are easily typed on common keyboards. Choosing uncommon characters in programming languages, such as en-dashes and em-dashes, is choosing to deliberately make our lives more difficult [1]. I think it's pretty clear from the article that his comments…
Well, the author himself is pretty ambiguous. While most of the article talks about code, the author mentions underscores being ambiguous in the context of underlined fonts. (I don't even think my text editor [for code] supports underlined fonts.) He also mentions only using shift once or twice per sentence when writing prose. So it would seem that the author is just hating on _ in all contexts. Which is kind of sill…
Well, mine does, and I used to have a co-worker who actually used it for coding. So there you go. :-)