After this BUR was made, I noticed that the wiki for bike shorts had the copeworthy sentence of "In some places, these are known as spats for some reason," which inspired me to check why they were called 'spats' in other places, because they obviously had to have some reason, right?
Well, it's because of the Japanese. Based on the following resources, and liberal use of machine translation, it became clear that the main reason why the term caught on was because the term 'spats' caught on as a term for leggings in Japan for one reason or another, with the kinds that only go down as far as above the knees being more narrowly called 'short spats', while full-length leggings were 'long spats'. There was even an attempt in the 2000s to try and correct this error in Japanese and refer to them as leggings again, but that failed and they stuck with spats.
This is all to say, bike shorts are not shorts, they are short leggings, which means they fell under legwear (if those tags were still a thing, but they aren't due to topic #21283).