Welp, mathecally is good. However, what you got there is an explosion whose diameter is 28.5 times larger than sun's circumference, and don't thinknthat at that point is equation is no longer valid. I honestly think that the one who di write that didn't known about the size of NY... or a pixel... or sun.
That calculation is for explosions made at ground level, and airburst explosion can also wipe certain level of surface. Since there's a few calcs that do not specify the type of explosion we always go with the low end.
Generally, anything that produce a shockwave is considered Mach 1, although that is not necessary true (20 psi shockwaves travel at Mach ~0.5). I do not calculate speed of explosions, but similar to non natural lightning/electricity, I wouldn't consider any explosion as fast as the one of tnt or c4.