I've always thought this is the most likely scenario, although it's really a range of scenarios (e.g. "has only a vague memory of what happened that night", "has no memory of that night but can't categorically rule it out because he was routinely blackout drunk", etc.)
I also think this can help explain the crazy Whelan/doppelganger episode. They know it can be established that he routinely drank to excess and that will render his refutation of Ford's claims less reliable. So there's a cockamamie effort to "prove" he didn't so it.
It's also almost certain that Kavanaugh is lying about never having been blackout drunk.
