You're probably aware, but Stern's written warranty has never covered they playfield. Not back in the day of LOTR, not last year, and not now. Go read it -- it covers three things, and three things only: CPU, boards, and display. That's it. So this new disclaimer language does nothing to change Stern's written warranty, which already excluded the entire playfield.
So what does this disclaimer language mean? It seems to be putting in writing what Stern's in-practice policy already was. As home sales have increased, they're getting claims they never would have before (an operator in 2010 was not going to try to return a playfield over insert ghosting). They've seemingly adopted a policy of allowing for returns if there are "major" defects on a playfield (more than X ghosted inserts, significant warping, etc.), but not "minor" defects (mild ghosting on just a few inserts).
Should Stern figure out why they're having all the ghosting problems? Heck yeah. It's costing them a lot of good will, and presumably a lot of money doing playfield swaps. But the new language on the front of the manual doesn't seem to indicate any change to their "unwritten warranty" policies.