That assumption doesn't make sense to me, Dave. In the posted photo, the delamination is creeping across the entire surface of that panel, right up to the edges of the seams. And the pattern of peeling looks exactly like the delamination I've seen on many other old PU coated articles like tent flys, jackets, and drybags used for soft stuff like sleeping bags. Some polyurethanes (polyethers) are more resistant to this kind of degradation (hydrolysis) than others (polyesters), but all PU coatings are susceptible. The fact that people consistently have this problem with Marmot gear suggests that it is at least partly attributable to their choice of PU material or their manufacturing methods.
I don't have any opinion about Eric's options in dealing with Marmot, and he might or might not have had abrasive things in that pack. We don't know. But it looks exactly like typical PU delamination that can occur without any abrasion. Given the information we have right now, your assertion that the damage was probably due to something Eric did doesn't make sense, and frankly it sounds like the kind of imaginative excuse often invented by warranty departments to avoid being accountable.
If you have some discomfort in general with placing any blame on Marmot, and you're looking for a fault in Eric's claim, I can see several arguments that are better than the abrasion excuse. For example, it could be argued that, since degradation and delamination over time is such a well-known property of PU coatings, people should expect it when they buy PU coated articles. In other words, like food that will eventually spoil, PU coatings should be assumed to have limited lifetimes. It is forseeable, inherent to this class of product, and not a defect.