I've never met Mr. Austin, but I've developed a good deal of respect for him and for his opinions based on what I've read in the past in this forum. However, I must point out that there is an error in the link to his site that you have provided, specifically where it states "restriction ring adds difficulty similar to that which would be encountered welding a butt joint on a T, K, or Y connection".
By definition any TKY connection is a sort of T-joint and is not a butt joint. The reason that the D1.1 6GR test looks the way it does is for economic reasons - it would be prohibitively expensive to make a TKY mockup for every welder who needs to test. The restriction ring helps to approximate the access problems common to actual TKYs.
Best regards,
Mankenberg