SEC documents show that Marvel was in development for a Namor movie with Universal circa 2006. We also know Marvel HAS licensed the character out and they have mentioned that the movie was being worked on in official documents.
https://www.sec.gov/Archives/edgar/data/933730/000111667906002593/ex99-1.htm
https://www.sec.gov/Archives/edgar/data/933730/000111667907000580/m10k.htm
Universal just didn't pull the trigger at the time and ran out of time much like they did with the Hulk.
I have recently theorized, based on SEC docs, that the Hulk distribution rights revert back to Marvel in 2023 or 2025. I think Namor may revert at that time as well if the contract signed was similar. As to why Marvel Studios still haven't used him, who knows? They didn't use Nova and between the Guardian movies and Infinity War/Endgame they have had plenty of opportunities to use him, more so than Namor IMO.
I don't think they'd have been able to license him at the time if your theory was correct, and we would have seen the issues you are alluding to in the documents. However, we can't know for sure.