I know that threadless hasn't officially announced that the long sleeved tees that they're printing on are their own brand already, but I really think they are for two reasons.
1. If you look at the size and garment information page, for every type of clothing is says what brand it is except for the long sleeved tees.
2. I noticed that the long sleeved Caged tees are on Charcoal... and I don't remember charcoal as a FOTL or AA color, but it is a Threadless brand color.