Going to be working on means latter. The Nerf is now. So he's saying DO NOT USE A SHADOW TANK NOW. They are worthless until we balance bosses, which isn't happening in this patch or any time soon (as there is no eta, and they are not even sure how they need to approach it)
Any reasonable Dev team would have included in that answer "that is why we have decided not to go ahead with any changes to tanks until we have have had a chance to rebalance boss fights and ensure they are in line with expectations first"
What we got was "nerf first, ask question later. For all we know this change is going to complete gimp you vs. today's content (in fact that's what our internal numbers are saying) but you're just going to have to deal with that until we change raids. Why do we care if you spend a few months unable to tank?"