For example, The Selfish Gene and Advanced Algebra are two very different books, but they can be both categorized as nonfiction.
Maybe you are right with the conclusion, i.e. CNN or TYT should not be both categorized as “Partisan Left,” but you need a better reason.
I think TYT is much further left than CNN (to an European, CNN is actually center-right). So, if you want to study whether there is a "funnel" from the center to the (extremist?) left, then you have to distinguish them, otherwise your results will be biased (at the very least, biased towards right-wing extremism).
Second, we don't know if YT recommender treats them differently or not. There is some evidence that it treats "mainstream" channels (i.e. TV stations) differently than "alternative" channels. Whether TYT falls into the latter is not clear.
To categorize them both as “Partisan Left Focused on politics and exclusively critical of Republicans” is a stark example of why manual labeling of these channels, especially by the two authors and some unnamed person the authors trusted (pg. 4), cannot be unbiased and a survey methodology or topic model would produce more persuasive results.
Maybe the lines are now blurred, but it would be interesting to see the amount of bias in the traditional daily News segments. Obviously the commentary segments are much more biased.