The drugs definitely got to him, but he's not the only artist that's said stuff like this. Nas said a couple years ago that he doesn't remember what songs are on which albums or how they're sequenced, and doesn't go back and listen to them. "Lose Yourself" is Eminem's most well-known song, and he didn't remember recording the original version of it until Paul Rosenberg told him about it. He didn't even remember being in the "Still D.R.E." video, but Snoop did.
Wayne's been consistent about not remembering things. He didn't remember the "Lollipop" remix, even when the verse was being rapped to him. He told Eminem a few years ago that he has to look up his own lines because he most likely used the line already. The more material you have, the harder it is to remember. And in Wayne's case, they're just songs to him. Verses he probably rapped over different beats, and they were probably just one-verse songs that were put together with other verses to make new songs. He'll remember songs/albums he grew up with and influenced him, but not his own work.