Ok, so someone uploads a mod made a decade ago (like a lot of these mods). The modder has since left the community. Some random joe takes the mod he downloaded and uploads it to Steam Workshop and starts making money off of it. What now? The community knows they didn't make it and report it - all Sega can really do is take the reports at face value, or ignore them because there's no real way to legitimately verify if that user made that mod or not. Then Sega's got a niche audience of romhack players mad at them andpeople who didn't make mods potentially making money from them. It's lose-lose. It's not smart to monetize a pre-existing modding community.
So, in your example, the obvious solution is to take the mod down and let the user who uploaded it try and fight the take down claim. You know, just like how all user-uploaded websites work. The "better safe than sorry" model that all UGC-websites follow.
The scenario you propose is not difficult to navigate at all.