A link to the brain doesn't matter if the brain is compromised due to a massive combat knife running through it. If a Forerunner's brain is similar to a human's in composition, then that maneuver would have cost the Didact motor functions (ability to control your movements), a lot of memories, and possibly even some of his speech skills, among numerous other things. Adrenaline may be one hell of an anesthetic, but it doesn't negate complications from injuries. In summary, the Didact should have either died or been rendered brain dead from this encounter. No if, ands, or buts about it.