I don't follow your logic. Yes, CNN and TYT are very different. This only means they can be tagged differently, but it doesn't mean they cannot be tagged under same category.
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 the more distinct categorization is relevant in this particular paper.
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.
In my view it’s a good example of why the manual labeling is flawed as CNN regularly criticizes progressives and airs republican views that TYT then responds to and criticizes CNN for.
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.
Iirc, stations like FoxNews and CNN have (had?) segments of traditional breaking/daily News, and segments that are commentary.
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.
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.