I do not think this has anything to do with merging the conferences. They will still be three separate conferences.
What an alliance would mean is that they would control college football.
If they agreed to only schedule games within the alliance and exclude playing any sec team either in season or bowl games it would cut the sec off at the knees. They would also be incomplete control of how any playoff would be constructed. It would also force ND to join one of the conferences because they could close them out completely.
Probably be the best thing at this time for college sports. It would allow them to eliminate the ncaa.