Wait a tic..I used air blockstun rather than ground blockstun because I can't read tables. 5C is actually 18 frames of blockstun, so it's 10 frames until Dead Spike it out, so you should actually have 1 frame to hit him out.
./training mode time
Yea, just tried it in training mode, and you can hit him out of it without IBing. It's just awkward because of how long you're in block stun from 5C.