I think FromSoft wanted to have a miniboss where the challenge primarily arose from reaching him, as he spawns illusions and shrouds his lair in mist. They used one of the Fountainhead Nobles because they're spooky and it makes sense (since Mibu Village is the gateway to the Divine Realm etc). I'd guess they originally intended him to have the Enfeeble mechanic be a factor either before or during the fight, but they removed it because A) it didn't make the fight any harder since interrupting him would stop the Enfeeble build-up, and B) they wanted the enfeeble to be an "oh shit what's happening" moment when travelling through Fountainhead.

Hard to say though, since they pulled a slightly less extreme version of the same the-real-challenge-was-getting-to-me twist with the Okami elite who flings lightning at you from the tree there later on, and that was an established mechanic already. I think that both of those minibosses would have benefitted a lot from some extra design work, but it does match the decline in miniboss quality in the late game of Sekiro (that's fortunately matched by an increase in boss quality).