It seems relatively clear that you owe them 5%...
You agree to pay Epic a royalty equal to 5% of all worldwide gross revenue actually attributable to each Product, regardless of whether that revenue is received by you or any other person or legal entity, as follows:
a. Gross revenue resulting from any and all sales of a Product to end users through any and all media, including but not limited to digital and retail;
b. Gross revenue resulting from any and all in-app purchases, downloadable content, microtransactions, subscriptions, sale, transfer, or exchange of content created by end users for use with a Product, or redemption of virtual currency, either within a Product or made externally but which directly affect the operation of the Product;
The first line of the agreement seems egregious though, lets say you make a moddable game that you release for free. Somebody makes a mod and charges $100 for the mod and they get the revenue. It seems like this is saying that you, the developer, owe $5 for each sale of that mod. I don't see that as enforceable.