I agree with this, 100%. The red/yellow/blue on the back cover are pretty rich and vibrant. That wouldn't be the case if the cover was printed poorly, with low ink levels or anything like that. So, I'd conclude this copy was stored out in the open (maybe on top of a stack of books) and exposed to UV over the years.
Structurally, I have this copy at about a 6.0. But, the Overstreet guide suggests fading is only "allowed" at 4.0/4.5. I'll split the difference, and call this a 5.0. Unfortunately, if you're looking to sell, the reduced eye appeal on this one might make it a tough sell.