The critical damage effects happen after you fail a critical damage save.
So if a kulworm bite brings you below 0 Grit, it triggers a critical damage save; if you fail, the kulworm swallows you and you need to make a STR save to escape or take additional damage every round, essentially leading to a death spiral. If you succeed the initial critical damage save, you’re still on your feet and not swallowed.
Hope this helps!