I mean, suppose the computer has WiFi IP address 10.0.0.3 & Ethernet IP address 10.0.0.5, then after NAT the return packets will go to 10.0.0.3, and therefore should go to the WiFi interface, not to the Ethernet interface (or, if they don't, how do they know which interface they should go to?).
I understand how cross-interface packets can be used maliciously. I'm just trying to figure out the non-malicious use cases for them.
The server also runs some service, say ssh, and you have a name for it in the DNS that resolves to one of its IP addresses. When you type "ssh vpn-server.example.com" it should work regardless of whether you're in New York or London, right?
If 192.168.0.42 can reach 192.168.1.42 by routing through the VPN server then it should generally also be able to reach 192.168.1.1 on the VPN server itself.
The described attack utilized a malicious router.
I imagine, in theory, that any middle router (such as your ISP) could then be used for such an attack. Imagine Comcast being able to inject their garbage [0] into even VPN sessions. Or a government actor that Comcast is known to route for.