I appreciate the author raising some awareness about string.join (I didn't know it was there), but I think his application of it is a little ill-conceived.

In my opinion, string.join would be most appropriate if you alrady have a string[] for other reasons.  If you just want to concatenate strings, using StringBuilder and iterating across all the items will be more efficient most of the time, because it takes time and memory to allocate the temporary string[].