why does"less" go with a comparative form?

The statement itself is not proper English at least. Since I didn't know the length of either river I went and checked--the Yangtze being 6,300 km and the Yellow being 5,464 km it is a true statement (referring to the ones in China), it just needs to be restructured to be more readily understandable. I would suggest "The Yangtze River is not in the least shorter than the Yellow River" or, better yet, "The Yangtze River is certainly longer than the Yellow River."

It is not shorter for sure , it could be the same length or longer ,you can find out from the context of the paragraph .