I looked at the video several times and he really slammed his helmet into the turf. So yea it could well have been both things... but I believe he was knocked out cold. Not sure sternum hit would do that but don't know for sure.
Yeah, I don't know either. Just falling like that with a helmet on I would think/hope it should prevent a head impact that would knock you out. I remember taking a hard hit to the chest and things went black. I was thinking if he a hard impact to his sternum it could have caused it. Though they just showed a replay and his head/helmet bounced off the turf. You are most likely right.