I think a big part of your problem is that you are assuming from his quote that he made them useless. There's a pretty big margin between 'stupidly effective' and useless.
Have to put my two cents in regarding this issue, since it is a pet peeve of mine.
Pet classes. I was a mage in EQ. That was a pet class. If the pet died while I tried to solo, I usually died. Pet chaining was difficult, timing based on mobs, etc. In most mmos I have played since, the pet classes are "classes with pets". Where the pets are little more than active dots, them dying does not mean your death, as usually there is more than enough skills left outside of the pet that will keep you alive or even be able to kill the mob. A pet class with a pet that does little to nothing expect deal out a little bit extra damage is not a pet class. A class that can solo without the pet out is not a pet class. IE: Mage in EQ had pets. The pets were used to control/survive the mobs you were fighting. It was a pet class. Necro in EQ had pets, but controlling mobs and survivability came from kiting/snare, not a pet class. Shamans in EQ had pets. Used slow + roots control/kill mobs. Not a pet class.
If there is an intended pet class, and its survivability relies on the pet, that is good class design. Mage groups in EQ were a thing (though preferably with a healer), but the places where it was viable were rare as higher tiered mobs hit much harder than what you could heal the pet for alone. A mage group could, however, chain pets, at which point it became a mana race. However, players should be able to learn the world, and the mobs in it, to where they might be able to utelize their class best. In EQ after PoP you could see necros constantly soloing in the Halls of Heroes or whatever it was called. Kiting. Wizards were AE kiting giants. I could not solo those places effectively as a mage, but I could solo other places. You learned from trying and from getting tips from others in your class. You are X class, Y area is great for you.
Outright nerfing of a class/ability from ONE encounter seems very strange to me, unless every mob in the world is similar, and one fight is the same as the next no matter the mob. That way you would know that if a class ability seemed overpowered against one mob, you know it is so against all of them, but that is not bad class design, that is bad world design.
I do not know the circumstances that led to the nerf or just how overpowered the group actually ways, but again, nerfing based on one expericience seems overboard. If they intend conjuration to be summons, and have pets as the main focus, then rather than nerfing the pets, nerf the actual player abilities to where it becomes a case of pet dying = player dying. From that one dev post alone, I now am fairly certain that this game will do what other games have done. Pets will be simple dots, they take aggro from you, but if you get aggro, it makes little difference.
Nerfing is needed if you have a player with the abilities to survive without a pet at the same time as they have a pet. That is an overpowered class. Nerfing the pet and not the player though is bad design when it comes to even talking about pet classes. So, it does not sound like ESO has a pet class. It has a conjuror with temporary pet dots that also work as a minor CC. In Skyrim you could be a 2h warror with plate + have conjuration at 100 with the Daedra knight or whatever the pet was called + restoration and heal. Pet alone close to oneshot dragons. If ESO even comes close to letting you build up a class with a similar power level then nerfing of a simple pet will do little to fix that.
Closing comments. Know little to nothing about the game other than it is F2P right? So I will try it out eventually. I like running around in Elder Scrolls games.
(edited in some filler examples)