This statement tells us he's done this so many times, he doesn't remember that particular encounter. Anthony Rapp is (to him) just one more minor who got lost in the shuffle. Hopefully some of Spacey's other victims will now come forward.
He’s creating something of an excuse for his disgusting behavior in being drunk, too, as well as a way to avoid admitting guilt by claiming no memory. Look for a “confession” of a long standing, hidden problem with severe alcoholism to the point of blackouts in the very near future, at which point he’ll disappear into rehab.
Hope so.