Armed aircraft can only be certified by a military authority. The FAA cannot certify the AT-6 like they did the T-6.
The USAF has not purchased AT-6s in any numbers, so they have not certificated it, either.
The fact that it is uncertificated in the US is a factor in evaluations, but is not driven by the lack of ability to be approved, which is how I read your comments. Please correct me if this impression is incorrect.
When the bids for the two aircraft were submitted to the USAF in 2010 for the LAS program they were required to present a cost/schedule plan for obtaining a US military airworthiness certification (the A-29 already had a non-US certification). Ultimately the A-29 won because, according to the USAF, there was significant technical risk to the AT-6’s certification plan. Incidentally, this appears to be the same reason the Philippine Gov’t selected the A-29 over the AT-6. The USAF hasn’t procured any AT-6 aircraft. Does this help?