That's very interesting.

The relevant paragraph from the standard is in ISO 8601-1:2019 §3.2.1:

> All characters used in date and time expressions and representations are part of the ISO/IEC 646 repertoire, except for “hyphen”, “minus” and “plus-minus”. In an environment where use is made of a character repertoire based on ISO/IEC 646, “hyphen” and “minus” should be both mapped onto “hyphen-minus”.

And as you correctly state, Unicode is based on ISO 8859 which is based on ISO 646. So it would seem the intention really is to use U+2D hyphen-minus when Unicode is the character set.