I feel like Sindel is a invasion boss because NRS ran out of ideas for invasion fights. She was a NPC in the story mode and it would be easier to reuse her for a invasion battle.
It's not like they need to have ideas to the invasions. Kitana once was leading a Orderrealm invasion, like, wtf she does have to do with the Orderrealm?! Nothing. They just throw some characters in it and this is it.