You put the emphasis on the wrong part of the quoted documentation. The essential part is has ever been used in a numeric context. Either side of binary + is classifies as numeric context.
Indeed that would have been right part to explain the difference re what happens when you uncomment the line with the binary +: in fact it's the one quoted by the author of the original article. (But I didn't want to repeat). However the one I emphasized did apply to what I was discussing around that point i.e. why ++($f='z') is 'aa' and ++($f='9') is 10.
