That is contrary to law.Agree in general, but I assume what's happened is he's committed a YC offence (which we see in the video, stonewall yellow, don't know what the #3 has lost his head about at first!), the referee has gone to note the booking and then realised at that point the player isn't on the team sheet. Which arguably mandates another yellow for entering without permission?
I've refereed in leagues without team sheets, so it can't be the case that "being on the team sheet" is what defines you as a player. If he's been sent on and it hasn't been spotted by the AR or a team official at that point, he becomes a player when he steps onto the pitch - after which, if he's sent off then his team will play with a man down and the sub can't just be reversed.
For me, he's definitely a player. The question is, what constitutes "permission" to enter the FOP? The AR will have waved him on, but is that valid to do if he's not on the team sheet in the first place, or does that omission mean he can't ever properly have permission to enter during the game?
"Anyone not named on the team list as
a player, substitute or team official is an outside agent"
It is clear from that statement we cannot treat this person as a player. And as such he should have been removed and the offending team should have been allowed to continue with 11 either by the player who left returning or the substitution procedure being completed properly with a named substitute entering the field of play.
I do wonder how this happened? Was it just an omission or did they have one more player than would have been allowed in the team sheet.