Xfinity Stream for AppleTV bug?: "This device can only access Xfinity Stream on your in-home Xfinity Wifi" error when on home network

I get the above message when trying to sign into Xfinity Stream on my Apple TV (4K, Third Generation (2021), tvOS 18.1).  I am on my home wifi (Orbi wireless mesh network in bridge mode to XB8 modem over ethernet), and I have a TV package.  The cable modem can see the AppleTV and has provided it an IP address.  The AppleTV is unable to access the cable modem directly over wifi because of the geographic layout of my house and the needs of my network (hence the Orbi mesh network). 

Is this message trying to say that Stream on AppleTV will only work when connected DIRECTLY to the wifi provided by an Xfinity router? That's a....frustrating limitation to say the least, given the drawbacks of the XB8's wireless hardware. The AppleTV has lightning-quick network access for every other service, so the mesh network is working as intended.  Running a separate wifi network off the cable modem for this single app is not acceptable for my situation (and doesn't work anyway given the geographic layout).

There are no hints of any of these concerns laid out in the AppleTV eligibility and requirements page (https://www.xfinity.com/support/articles/stream-on-apple-tv-overview) I've read the FAQs here (https://www.xfinity.com/support/articles/stream-on-apple-tv-faqs) and they seem to hint that it's true that the AppleTV needs to be connected to the wifi network provided by the cable hardware.  But what if I had an AppleTV with an ethernet jack and connected that to the cable modem directly?  Would that work?  Why or why not?  Because that is precisely the logical layout of my network:  the mesh network is a wireless-to-ethernet bridge for all my devices.  Indeed, the cable modem reports that all my devices to it are connected to it via ethernet (the cable between the modem and the mesh network).

As with many things involving Comcast, this is all very frustrating.  The messaging is incomplete, the limitations are arbitrary and hard to understand: I can use Xfinity Stream on my iPhone on any network--why not AppleTV?