I seem to recall that this came up a few years ago and further research by a few individuals suggested that Costco was not an authorized dealer, implying that the warranty would not stand. I don't know for sure if this is firmly the case but a quick check of the website yielded no results for Patek Philippe and so perhaps your sighting was a one-off. I do agree with you about the effect it would have on the brand, and, given the demographic targeted by PP in its advertisements*, I suspect they would agree with you as well.
*To PP the 24 is a PP like any other in its collection and any degradation of brand value (I would hope and expect!) would be taken extremely seriously regardless of whether the watch was quartz or mechanical, steel or precious metal.