So let me see if I understand this...
...they have to be close to the sub to detect it, but they want to attack it from far away?
So what range do they release the weapon? I know, that answer is classified. But consider this:
1. The travel time from time of release to the time the weapon hits the water.
2. The target is moving.
3. The sonar on the torpedo has a limited range, so
4. The torpedo goes into a search pattern to acquire the target, but has only a given amount of fuel on board.
Now, if the aircraft crew still has a fix on the target via the sonobuoy pattern, and can guide the package, that may improve things a bit. That was the advantage that the Ikara had over the older RUR-5 ASROC. But it depends on maintaining that contact with the target.
Now given that a good submarine sonar operator will here a P-3 at low altitude coming in for a drop, the P-8 is doing the work at high altitude. (The operator would hear a TU-142 at altitude though)
Seems strange. But with network-centric warfare the sensing platform is often not the “shooter”. Just another tool in the toolkit. It sounds like the winged version of the small diameter bomb used in precision, high-altitude land-attack. Putting wings and a guidance system are like putting a scope on a hunting rifle — not a huge expense to add a capability.