I've heard the solution to that is to set up an AP in your house that is either firewalled or doesn't have an outbound connection and connect your smart devices to that AP.

Apartments are a problem though

Unless the TV ignores your settings and hops to random networks when it can’t phone home through your AP, this approach should still work fine in apartments.

From what I understood, this is precisely what some new models do.

Why would they even have such logic? Especially since the only open wifi points are captive portal ones?

But if there's just no open wifi points around then you can still be pretty certain of this.

Buy a cheap router, don’t connect the router to the internet, and connect all smart appliances to the router.

Wonder if the TV would detect it as nonfunctional and still search for open networks that work.

If you can think if it, they can too.

As Samsung is essentially hostile beyond belief, and has thrown away all pretense of 'helping', I'd say yes.

I wouldn’t be surprised if they did but isn’t it illegal to connect to an open wifi if you haven’t got the explicit permission to do so?

All they need in that case is a signed contract with Comcast saying they’re allowed to connect to Xfinity networks.

Or buy a cheap good one ( eg Edgerouter Lite or X) and block them there, or put them on their own subnet.