Hopefully this isn't the last we see of this, but I'm not holding my breath...
As for the antimatter bomb, I feel like it was less powerful than it should have been, but not by much... afaik, 1kg of matter that gets annihilated with 1kg of antimatter in a perfect matter to energy conversion would have an output of 180 petajoules, around 43 megatons of TNT, a bit less than the Tsar Bomba yield of 27,000kg (damn that energy efficiency sure is crazy huh).
Still, unlike an hydrogen bomb, and I guess thanks to magic, the area of effect gets concentrated to just the city's border, liquifying everything within its range... though I guess in this case it would be more like crystalizing.
Wish we could see more of this kind of mix between magic and science, since things can get pretty crazy with weird materials combined with the law breaking properties of magic... Planck matter sword anyone? It only has a weight of 5.155 x (10^93) g/cm^3, slap in some anti gravity magic and a bit of space-time correction magic and you're good to go! A single swing can effectively annihilate an entire planet! Not OP at all.