The union invades a fortress that has 2 CSA SPs defending. The combat result is 2 for the attacker and 1 for the defender. The 2 CSA SPs are eliminated. But is the Union forced to retreat? I refer to rule 8.8.6 regarding the additional intrinsic force that would remain in the fortress.
I think 8.8.7 is the rule you actually want. That's the one that says that all defenders (intrinsic or otherwise) need to be eliminated for he Union to avoid retreating. 8.8.6 merely requires the Confederate player to take actual SP losses rather than taking intrinsic defenders instead.