Per game mechanics, barring a bug, it is impossible to merge every region into another realm. You have to leave one city behind. What happened here is that every region of a realm merged with another realm, to the extent possible by mechanics. (Again excepting bug abuse.) The realms involved are friendly. They were in fact strong allies. The merger provides a big advantage in a war to the realm to which received the regions; economically, militarily, and geographically.
What you have here are two regions who have historically been strong friends and allies. They are both strong, viable realms. Neither one is under any obvious external pressure or duress. The action taken merges the two realms, providing a large and obvious advantage to the realm that received the regions. The realms were merged to the maximum extent possible, limited by game mechanics. The remaining region is a city that cannot feed itself, and can only exist by receiving food from the realm to which all its regions moved.
Whether or not there were any IC reasons for the merger is irrelevant. This is an ooc rule which limits the actions of players for ooc reasons. (Besides, anyone with two braincells to rub together can always come with an IC reason to do anything they want. )
Imo, this violates the realm merger rule to the maximum extent possible without deliberate exploitation of a bug. Even if the player that did it did not intend it that way, That's how it ended up.
