Actually, it kind of is. The rift between story and fact is enormous here. While Halo is fictional, it has always been grounded in scientific fact. If we abandon that, then we abandon a core principal of the Halo story writing process. Cool factor cannot and should not come before cold hard facts.

Perspective is a very large role here. It really depends on A. Length of knife B. How far the knife went And C. What's the extent of self healing in forerunner armor. I wouldn't be surprised if forerunner armor can reconstruct damaged muscle tissue and organs, but that does still leave us with memory loss, etc.