There's a lot more to anti-shipping salvo generation than just pulling the trigger. Without effective coordination and synchronization, munitions end up being employed in piecemeal, scattered fashion, which absolutely plummets salvo pA.
Civilian merchant shipping will likely still be present during the (short) window that TW will be able to conduct anti-shipping missions, which creates the issue of not only civilian collateral casualties (which I personally don't regard as a concern, but we do take it seriously), but wasted munitions that end up prosecuting civilian vessels.
If it were as simple as push button -> missile launch, this would be true, but again, there is a whole lot more than goes into the whole ASuW process.
Granted
But if they're talking about scenarios with 1000+ missiles, you would think there would be enough missiles to saturate a small target box in the Taiwan Straits, no matter how messy it gets.
I think we can ignore the silly assumption that China would launch an invasion straight away, and assume that this occurs when there is no purely civilian traffic in the Taiwan Straits.
Hence my view that a submarine with 2500nm ranged ballistic missiles is better used in the Pacific, rather than within the 2IC where China can already concentrate lots of force.
From the Northwest Pacific for example, it could target:
1. The bases at Alaska or Hawaii, and therefore prevent large concentrations of bombers or tankers being deployed
2. The naval bases at Seattle and San Diego
3. The B-2 stealth bomber base in Missouri
4. The B-1 base and F-35 plant in Texas
etc etc
If Chinese missiles can attack targets in the Continental US, then it becomes a far more serious decision for:
1. the US to go to war with China on behalf of Taiwan or
2. the US deciding to attack targets in mainland China