A thought experiment: as far as I am concerned, the statement "ISO 8601 mandates that you write dates as YYYY-MM-DD" is nothing more than a hearsay
I haven't read the document, I don't know of anyone I trust who have, and so from my perspective it is practically just a 62890-hand information which may or may not be true.
(YYYY-MM-DD is still a useful format regardless of whether it's actually an International Standard™ or not, though)