Based on the simulations I've seen, yeah. I'm sure there were plenty of panic in the sub and during the anaphylaxis, but I believe once the sub failed, there was less than a second between in first physical sensation to the complete disorganization of the nervous system, rendering the sub death quite painless. With anaphlaxis, even a sudden, sever attack, there will be several minutes of (at least) muscle strain as your diaphragm desperately tries to pull in more oxygen, and also general pain as your tissues squeeze against one another as they expand and nerve cells die.

I can and have accepted death; I'm too old to believe radical life extension will save me. But, many deaths are incredibly painful. If I have a choice on how to go out, inert gas asphyxiation seems best, but some sort of rapid disorganization isn't too bad. Anaphylactic shock seems worse.