Who said there was a head injury that precluded wrestling later in the day? You can get your "bell rung" after a slam and yet be able to physically pass concussion protocols 5 minutes later. Just because the ref calls for HNC timeout doesn't mean that the wrestler has a concussion. It just means he needs to be evaluated by medical and pass protocols before continuing. If he passes protocols then he can continue in the match or in a following match.

The problem with a slam is that they may take 4 minutes to pass protocols to continue or to recover from any other type of injury. If it was a legal HNC injury then they could return after that 4 minutes of protocols. The problem with a slam is that you only get TWO minutes of recovery time, not the FIVE minutes involved with a legal HNC injury.