I think they just mean hyphen as the high level meaning of the word (in opposition to slash or period), but they are not advocating any exact character codepoint, like U+2010 HYPHEN.
If you copy this sentence from the given ISO 8601 source itself, it’s the U+002D HYPHEN-MINUS that they use between square brackets:
[-] (hyphen): to separate the time elements “year” and “month”, “year” and “week”, “year” and
As @yindesu said, they always use HYPHEN-MINUS character to mean hyphen.
And MBS, for the moment, has also chosen to use HYPHEN-MINUS for dates (2013-07-14).
While MBS does use smart character EN DASH for spans (tracks 2–21).
ISO chose “-” instead of the natural “/” or even “.”, for file name computer reasons so I think it’s good to keep it easiest possible for programs like userscripts to parse dates, etc. to stick to the plain HYPHEN-MINUS, that will always be typed by new editors, and can never get down to 0% usage.