Indirect revenue can mean anything from merch sales to concert tickets to selling your online course, whatever it is. Indirect revenue can literally be anything.
More importantly, just because a creator is not monetizing a piece of content does not mean that the content is not being monetized. On a lot of platforms, ads are run on content regardless of the creator monetizing them. That content is, by its nature, getting direct or indirect revenue. Literally everything on YouTube has direct or indirect revenue.