It's always been that way, at least in their current gen trucks, 6.2 has been available on all their upscale trims. AT4 is GMC's version of the Ram Rebel and the Chevy Trailboss, it's more offroady. AT4 has been their since 2019 as well.
My point with the 5.3 vs 5.7 vs 6.2; for the money I spent on my truck with the 5.7, I'd still have had to pay about $10,000 more for a similary equipped GMC SLE with a 5.3 with their crummy 8 speed. So that's what I meant by "underpowered". Why pay that much more, for a truck that has less power than mine? What I really wanted from GMC was the 6.2 and 10 speed, but then I'd have had to step up to a more premium trim (paying on top of the already $10k difference to do so), then also paying another upgrade fee on top of that for the 6.2 engine itself. I just couldn't justify that cost. Again these are not MSRP prices. Back in 2019 when I bought mine, dealers were offering cash and incentives on the hood, it was a totally different time.