I believe it means episodes where all the cartoons follow a similar subject. For example, some contain shorts all revolving around Horror, or musicals, crossovers, U.S. politics or foreign country settings. I didn't create that category though, only the subcategory.
I think a Themed episodes is more of blanket statement. What you called out in your return mail is more of what should be used: ie > Horror, musicals, crossovers, or U.S. politics or foreign country.
Granted this get very nit picky like this one has more blue and this more red. It just how small granular a person want ot get. And yes there a limit because I've seen people coming up all sort of items in a episodes just to get more edit credits.
Go a head with edits with the kind of category you have listed.