I suspect they do have in-house QA, although it is likely a small team, and even when they do identify a bug the Devs don't have sufficient time to fix them.
An opt-in test environment is great for testing new features to get feedback and it's the right move, you can't have an internal team determine if a new feature is great for the player base, it'd be like Arrowhead giving itself a pat on the back for coming up with a great idea.
Surely this is better than the crypto/NFT tech fad. At least there is some output from the generative AI that could be beneficial to the whole of humankind rather than lining a few people's pockets?