Yes, unless I'm missing a fair bit of info, he hasn't quite thought that one through.
It makes sense to me. You can diagnose a broken leg, harder to tell if a player has a concussion or the severity of it. At worst, a player hurts his leg, comes back and finds it is painful to run on and he can't play. With a concussion the player is dazed and not aware of how out of it or slowed his reactions might be. There is also some medical research suggesting that one concussion makes you extremely susceptible to another one, and multiple concussions in a short period of time might cause permanent brain damage.
The absolute worst that can happen to you with a knee or ankle injury is maybe you need to get a replacement when you are older. And I'm sure there are plenty of footballers whose joints ache terribly. But that's still not as bad as being reduced to a punch drunk.
If you follow American football, this is a real epidemic. Players would take a hit, see stars, the coach would give them some smelling salts and they'd feel like they could play. Then later they interview veteran players and they will say that they played multiple times after such incidents and don't remember anything from that game. They retire, and by the time they are 40 they start having all sorts of memory issues as well as emotional problems.
The number of pro wrestlers and NFL players who have in recent years committed suicide (or murder/suicide) or been reduced to shells of their former selves is quite concerning.
So I think that allowing for a sub because of a concussion reduces the incentive for player and manage to try and play through what might actually be a serious injury (or could become one). I don't know how they would prevent players from faking concussions or whatever, so it has some problems in practice. But it's an idea that to me is worth thinking about.