Correct, but if the snake started at the head, the quills would be forced backwards, the flat direction.
This is my theory. Not substantiated by field date of snakes eating porcupines. Just suggested by watching snakes eat mice, frogs, worms.
That seems to refer to actually swallowing the animal. The photo I saw didn’t seem to show any quills in the snake’s side, which could be explained by carrion (already dead) porcupine.
As far as American porcupines go, I’ve accidentally run over roadkill porcupines and had quills embedded in my tires. Very glad that they didn’t go into the sidewall instead of the tread.