Usually network interfaces will have a metric or priority e.g. route print will show in Windows or just route in Linux will show you. Mac being BSD based might be "netstat -r"

When you have two devices on the same subnet it should use the highest priority interface (usually the lowest number) when a new connection is created. Why Wi-Fi is prioritised is an Apple decision if that's what they're doing.

Also the other issue may be simply it already has a connection open via the Wi-Fi interface that it doesn't want to disconnect so it keeps using that. That should be fixed by stopping/restarting the backup or you could even try rebooting.

I highly doubt it's detecting LAN "hops" through Layer 2 networking devices like switches and access points (dumb or managed).