I don't want to beat a dead horse here, but that is true of new (or very recent) arrivals in the salvage yard. Once the doors/glass have been removed (usually one of the most frequently taken parts) or are already missing due to collision damage, and the carcass sits exposed for 6+ months the interior will go bad. This is more true in areas with extreme climate/weather changes.
For the record, I'm in St. Louis, MO - 100+ degree days in the summer, loads of rain in the spring and fall, and winters that can get so cold they are unbearable. This tears the insides to hell in short order if the car is wide open.
Also note that he's looking for a seat cover for a 1992 - this is a 13+ year old car now. Any 92 Deville that was wrecked with low miles (and therefore a clean interior) has been in a yard for years now, to sit and rot. Any 92 recently wrecked or retired (and put in a yard) likely has a load of miles (well over 100,000, at which point the leather is probably shot) so the seats won't be great anyway.
Finding a low-miles, pristine recently wrecked 92 (and thus a clean drivers seat) in a yard will be a very tall order, unless you live in an area with a LOT of auto salvagers, and a lot of Cadillacs.
Add to that: a lot of salvage yards won't take cars this old, and won't keep them around for long if they do have them. This is the difference between a salvage yard and a junk yard. The junk yard will have older (more than 7-10 year old) vehilces that are mostly stripped. The salvage yard buys wrecks that have a decent amount of clean usable parts for resale (how they maximize their profits) that will have some real demand, this means cars 5-7 years old or so.
Yes, there are exceptions, and a few yards have inside storage where they keep things like seats, but generally speaking, finding a mint (or nearly so) leather seat for a 13 year old car in a salvage yard is unlikely based on my personal experience and the items cited above.
Now, if he needed a seat for a 1999, I'd agree fully with your comments as there's pretty good chance of finding one of those in a yard with a decent interior.
Just my take on the situation, nothing personal.