No, what I was saying is that the "engine damage can't take engines lower than 1" rule does not impact the sniper turret's interaction with the focus fire control in any way. In truth, the engine health itself is taken all the way to zero, it's just that zero engine health can't take move speed lower than 1. Not 1%, mind you, 1.
Anyway, the sniper turrets would only "switch to" (i.e. consider) focus fire if every single hostile eligible for "I can shoot at this" either had engine-health == 0 or was immune to engine damage. At that point they should start focusing for damage if you have sniper-focus-fire on. Which is roughly what people want them to do in that case, though it gets tricky when more ships come in since they don't constantly update target lists, etc.