That isn't a friggin pinch flat! A pinch flat happens when the compression force is so great the tire and tube bottom out on the sides of the rim, resulting in small holes (not a slash) on each side of the tube, and it's often referred to as a snake bite.
OP: That's terrible luck, I know of a similar story personally. Best thing you could have done is use the rear brake hard and stay off the front brake. But to minimize these possibilities, this is where yearly maintenance pays off. Even if you don't have a flat all year, take off the tire and inspect the rim tape, make sure the spoke holes are covered. Also inspect the tire. When my tire wears out (which is the rear tire), I always put the new tire on the front and move the front tire to the rear. You always want the best tire on front. And again, periodically inspect your tires, tread and sidewall a few times each year. Really no need to periodically replace the tube - I've seen a few batches of bad tubes, so replacing a tube actually adds an element of risk. A new tube is really just as likely to fail (like a split seam in the tube, which you seemed to have) as an old tube. And I take it that wasn't your first ride on that setup, which pretty much rules out faulty installation. Not a pinch flat!