They should have included Thread support in all models.
Pragmatically once loose Ethernet then can have some problematical issues.
Thread
"... Thread is a low-power and low-latency wireless mesh networking protocol built using open and proven standards. Thread solves the complexities of the IoT, addressing challenges such as interoperability, range, security, energy, and reliability. Thread networks have no single point of failure and include the ability to self-heal. ..."
Thread solves the complexities of the IoT, addressing challenges such as interoperability, security, power and architecture requirements.
www.threadgroup.org
Can have a mesh network but if there is no backhaul back to the Internet all have is a totally isolated mesh to nowhere else. If the Wifi can't get a signal or isn't sufficient for backhaul ,then Ethernet is a better (more robust) option.
If the purpose of the AppleTV is to be a smart home hub then Ethernet is more prudent foundation. ( camera videos being streamed back and encoded-uploaded from AppleTV. )
There is likely other folks that aren't doing any Smart home at all. The AppleTV is playing the Roku/FireTV stick role only. May or may not want something deployed in that context to play a Thread router role. Also more likely to get this "video stream only" role with multiple AppleTVs where as just one if the house is probably sufficient as a Thread router. So Apple going for the more expensive "one home hub" and the lessor expensive spokes around the hub with the Wifi only model.
Deployment wise though most deployments are to single AppleTV and no Thread centric "smart home hub" . WiFi 6 is fast enough and more mesh routers around to blanket the dwelling.
Decent chance this has some tie in with directions coming from Matter.
Smart home interoperability matters, and Matter is making that easier.
www.androidcentral.com
But Apple also doesn't want to "give up" the revenues either. So doing gentle herding into the higher priced option.