Why? Binding to a network interface isn’t some dark arts. It is very normal and a very usual thing for programs using the BSD socket APIs to do. ‘Killswitch’ is expected to cover this.
The series of actions Android performs (when that “single system API” is invoked) to create a ‘killswitch’ would not look all too different to what one would do on a mainstream Linux distro with net admin capability / root.
What?! May be I misread what you two are suggesting… but just in case…
External hardware routers cannot act like a killswitch for an OS (without help from a locally operating software killswitch) because the external hardware is so far removed from the OS/Kernel that’s making decisions on network traffic, that the OS/Kernel might very well take a routing decision that also bypasses that external hardware router (for instance, connect via another radio).