Man, those factory seconds probably failed a stitch check or got a wrong dye lot, which is why they never hit a real store. Carhartt does small-batch runs, so a batch that gets flagged gets pulled and dumped in bulk instead of sold at full price. I've seen this at the Goodwill on South Willow too, bunch of Carhartt and Dickies with the tags cut or stamped. Check the inside label for a black marker slash or a hole punch, that's the tell that it's a second and not a stolen return.

30 years is a long time to be sweeping but that doesn't mean his one-liner is gospel. Nylon vs poly on glazed creosote, sure maybe it matters a little, but half your jobs being "not clean" sounds like a stretch. Did the guy actually show you a chimney he re-cleaned, or was it just a salty comment at the supply house?