Pretty weak reasoning. It just as often “or” like this/that. If not more— who’s actually looking at fractions that often? I’d argue the punctuation attached to that specific date format shouldn’t be the basis for the order itself, and dashes or periods are common too.

The better reasoning is that the day is typically more relevant than the month. A downside though is that it’s bad for sorting: YYYY-MM-DD is the best way to automatically sort by date, and ease in digital sorting is arguably the most important factor in date formatting. It’s kind of a silly thing that people don’t care about outside of memes otherwise.