Keep in mind that Primarchs have survived things like falls through orbit and then landing near the Earth's core (Curze himself managed this as you know), landings into volcanoes (Leman Russ), being shot by a Titan's city-leveling Plasma Blast Cannon (Lorgar, in Betrayer), and starship weaponry/macro cannons (Vulkan, in Fulgrim and detailed in a few other books). Then, when you consider that Konrad Curze was able to maul and cut up Rogal Dorn with his bare hands...