What the article is trying to explain here is that A=440 is just a convention. This is probably where the article should have had a discussion about perfect pitch vs. relative pitch. I would have written it like this:
“Since this note is 440 hertz, in Western music, because of convention, we call this note ‘A’. Now, let’s play a note that is 400hz. For most people, given enough time after hearing the 440hz note, this second note will sound like the first note. There is a minority of people with a talent called perfect pitch who would be able to tell that the 440hz note is in tune, while, with conventional Western music, 400hz is out of tune, because they are able to tell the absolute pitch of notes. Most people (including the famous composer Wagner) cannot do this: One pitch by itself sounds like another pitch by itself.”