All you need for discovery is broadcast access, and TCP/IP grants that. The discoverer broadcasts, "I'm looking for devices!" And the discoveree replies "Here I am!" Boom, connection is made. This is how game server browsers worked for LAN play back in the day.
Is it really so inconceivable that a phone is connected to a wi-fi gateway on a regular LAN?
Because that is how my network is set up here at home.
There are lots of added protocols (e.g. Bonjour) because TCP/IP alone is not about discovery.
And of course here we're talking about syncing over wi-fi. How would TCP/IP alone, without Wi-Fi enabled, cut it? It would sync wirelessly over ...cable ethernet?