I hate to be pedantic, but your sample size is really small and you make a lot of authoritative declarations. In my experience, that doesn't usually hold up well in the long run.
I should note this is not your typical GM that often makes mistakes, but a supervisor who spoke directly with the devs. So there is no mistake here.
Time will tell. We've been through that before.
This is basically Blizzard saying multiboxers have an unfair advantage
It does nobody any good to put words in their mouth. We've also had GMs/mods/devs explicitly say that it's a faulty premise that multiboxers have an advantage in PVP, and also that we have no more advantage than a group of 5 people on voice comms together and are actually at a disadvantage against any coordinated team of 5 brains assuming similar gear levels and such. I guess the number of us who remember these conversations are shrinking.

That said, *if* they decided they were tired of spending dev resources chasing/fixing/nerfing all the ways people playing multiple accounts managed to PVP successfully that the devs didn't expect, I couldn't really fault them for just walling off certain areas from multibox play. Then again, I also wouldn't go out and commit seppuku because I lost 5 accounts, two of which have been active for nearly 14 years. Caveat emptor, etc.