Then don't need to go that far, they just have to figure how soulcast things into uranium.
Also, the question of the power of anti-matter explosion is a hard one. Antimatter-matter annihilation creates a lot of gamma rays and a bit of neutrinos, the neutrinos are pure losses for the explosion but there shoudn't be a lot of them. The part that will create the explosion will be the gamma rays and the will do so in two steps, the first is ionising the air so it becomes a plasma opaque to gamma rays, once the plasma is opaque the following gamma rays will push the plasma and that is what creates the explosion, plasma pushed by light. It will be very hot but I have no idea how big it will be.
Anyway it's not like Rosharans can create actual vacuum, and neither can we.