Which is interesting because in the other universe they were gung-ho about continuing the cull
Were they really?
There were at dozens of Sathanas juggernauts in system, possibly hundreds. We don't know anything about the Dante's capabilities, but it was on the battlefield. And yet...
The total forces that attacked the 14th in Universal Truth were several dozen bombers and two cruisers.
It stretches my disbelief that out of all the forces in the system, the Vishnans somehow managed to tie down everything but strike craft and two cruisers. Especially when Shivans have such strong subspace capability. Doubly especially because the Dante was
right there. I can only conclude that the Shivans let the 14th go. They're playing the long con on the Vishnans.
From FS2 we can hypothesize that losses of Sathanas class are trivial to the Shivans, something that was confirmed in Tenebra. Yet in Fallen Angel the Sathanas shock jumps the Sacred Keeper, rather than any of the ships of the GTVA fleet. If exterminating the Humans was so important, why make that particular attack? Same for in Universal Truth, if juggernauts are disposable, why use them all fighting the Vishnans if the true goal is to cull the humans?
There's more to it too. In the version of the conversation the Vishnans showed to the 14th, they implied that the Vasudans were extinct and the Shivans wished to make Humanity extinct too. But that's an outright lie, because the Vishnans became aware of everything Bei knew when he joined them, including the continued existence of the Vasudans in the parallel universe. Now, being charitable, it might just be that these particular Vishnans and Shivans are only concerned about their own universe, but by allowing the Sanctuary to escape to the parallel universe the Vishnans essentially cause the same thing - no more humans in their universe.
And we know from later information (tech room and the second Universal Truth) that it's at least hypothesized the Vishnans and Shivans are pan-universal.