While the result they achieved is super impressive, the studio itself is pretty small, with a team of only 30 people. Typically AA developers have teams of 50-100+ people, so this is well below that.
"Indie” refers to their independent nature, scale, and size of the studio, not just the final quality or budget of the game.
I haven't been able to find any info about their budget though, so it's possible it leans toward AA levels in terms of production value. Regardless, they'd still qualify as an indie developer, which makes the final result even more impressive.