Well, I just checked forums, changelog, and discussions, and Semenar hasn't given any sings of life for about 3y already. You've kept this mod up to date, and you've also made quite a few changes, which I highly doubt had Semenar's agreement. The original mod is under MIT license, so, from the legal perspective you're in the clear.
I don't see any reason why you can't be considered the big boss.
Sure, maybe you don't want to bother because it's too much work, or you don't have enough time, or maybe every waking moment you spend it dreaming about how much you desire to see SerotoninTheft burning alive.
There are reasons why you won't do what I proposed, but the one you said ain't valid, pal. If this ain't your mod, then it doesn't belong to anyone.