Could have been simpler than that system. UPS does not have full face masks in most of their fleet, instead just separate masks and goggles. Considering that, they might just have had enough smoke entering those goggles to impair the ability of their eyes to stay open, or just watering too much to see. Cockpit visibility might have been good enough, absent that issue. No way to know without the CVR, maybe, but there is something to think about.