Someone doesn't understand his own work five years later to this extent, this is a strong indication that the work is actually garbage, and the prior understanding five years ago was only a delusion brought on by the circumstances: the late nights, the pressure, and so on.
Perhaps it doesn't make sense today because it never did, and the self deception has long worn off, not because the author has gone daft.
Several weeks ago, on the last work day before going on vacation, I submitted fixes for nine issues I found in one USB host controller driver. The last time I looked at the code was more than a year ago. I had refactored it and really improved its quality. Looking at some of the code now, I couldn't understand it that well. But that's because it wasn't as good as I thought it was. I was still relying on the fictitious story of how I thought certain aspects of the code worked really well thanks to me, and it wasn't meshing with the reality emanating from freshly reading it with more critical eyes. And, of course, I was also confronted by a reproducible crash. As I'm reading the code, I'm forced to throw away the false delusions and replace them with reality. This is because I'm smarter and fresher today, not because I've forgotten things and gotten dumber! It's taking effort because something is actually being done.
Perhaps a similar problem is here: he's reading the paper with more critical eyes and seeing aspects that don't match the fake memory of how great that paper was, which was formed by clouded judgment at the time of writing. Maybe that obscure notation that he can't understand is actually incorrect garbage. His brain is reeling because it's actually digging into the material and trying to do proper work, perhaps for the first time.
If you can show that your five year old work is incorrect garbage, that suggests you're actually superior today to your former self from five years ago. So that could be the thing to do. Don't read the paper assuming that it's right, and you've gone daft. Catch where you went wrong.
By the way, I never have this problem with good code. I can go back a decade and everything is wonderful. Let's just say there is a suspicious smell if you can't decipher your old work.
Good work is clear, and based on a correct understanding which matches that work. There is a durable, robust relationship between the latent memory of that work and the actual work, making it easy to jog your memory.