Monospace font, two spaces after punctuation, with a proportional font one space is used. As far as I know, as long as proportional fonts have been around, this has always been the proper way to do it. The reason we were taught two spaces in typing class is because those typewriters used monospace lettering (I am 46 so I had those classes in my younger days myself).
Using two spaces after punctuation with a proportional font you end up what is sometimes called "rivers of white" on the page. Justification settings also cause this effect. Basically, while looking at the page, you can see a white "river" going down the page formed by the extra gaps.
However, it seems silly to me to criticize people that are not professional typographers/designers for not following professional standards especially in personal correspondence. The guy has basically written an over the top "link bait" style article. No sane person really cares about the number of spaces you are using.
-Keith