Yes, it has problems. Its a narrow study looking at a small subset of tanking. It uses a debatable set of assumptions and requirements (like the no unchained among other things). It uses data that is available (fflogs) which has limitations of it's own. It's not a peer reviewed article in a professional journal. But its the best thing we actually have to work with. That's why my title is "interesting research" not "Conclusive proof that X hypothesis is correct".
I'm not here to defend the scientific efficacy of a bit of number crunching about a videogame made by a few people in their spare time. But it is still the only attempt at any tank analysis that goes beyond theory craft and why it should get some visibility, props to the authors, and take a look at what we as the tank community can get out of it. We aren't going to get journal peer reviewed studies. Comparing this to rigorous professional academic standards is really a bit excessive. It was never supposed to be that. Take it for what it is, not for what it isn't.
As for "useful" mitigation on AAs etc, I think this is an excellent example of why we need things like this to remind us to reconsider. We constantly minimize the impact of fluff mitigation in statements like this that which encourage people to ignore fluff damage because regens cover it and only mitigate spikes, yet at the same time bemoan lack of fluff mitigation as a balance point (and point to FF logs damage taken to support the claim). This isn't a peer reviewed study, but it is certainly useful in looking at how we talk about tanking and frankly, we do need to review how were talking about it because there are constant inconsistencies in what we do and don't value. We cant both say that defensive abilities giving healers extra GCDs is a powerful benefit of good tanking, then in the same breath push fluff mitigation to the side, and that is just one example.