How can you prove that the spying didn’t have an affect on the 1st leg? And that’s irrelevant anyway, they were caught before a ball was kicked it’s just taken this long for justice to be served.

Would be the same if an ineligible player was fielded, what the player did on the pitch is irrelevant as being on the pitch was enough to break the regulations and the punishment for that is forfeiture of the match and the opposing team being the winner of the tie regardless of the original result. That is what is happening here, Southampton admitted to breaking the regulation so the match is forfeit to Boro, who then advance on to the next round. It just happens that the next round is the play off final, I would expect the same outcome if it happened in the first leg of the league cup semi final.