watchOS iTunes Podcast syncing

I’ve started listening to podcasts on my Apple Watch with a Bluetooth headset. In the Watch app, it’s set to add episodes from the iOS Podcasts “Listen Now” list, as many as will fit. The syncing is interminably slow and only happens while the watch is charging (and the phone nearby) but I knew that.

It appears that when I’ve listened to an episode on the Watch, that fact is reflected in the Podcasts section of iTunes on my Mac but I don’t know how that’s happening. Certainly the Watch tells my iPhone what’s been played but I don’t let my phone and iTunes sync over WiFi so how is the information getting to iTunes? I do sync podcasts to the phone using iTunes but I haven’t connected it today yet the play state change is there. Is it via my Apple ID being signed in to the Apple Store? Is it a Bonjour networking feature?