This is alarmingly realistic to me.
The part about ISP routers broadcasting an alternate "hotspot" SSID is documented [1] and can be easily observed. Walk around any city, open your phone's wifi settings and see networks like "xfinitywifi" or "optimumwifi". As an end user, if you connect to one of these networks from your device, you'll get a captive portal web page that makes you sign in to your ISP account. Once signed in, you get internet access courtesy of the router sitting in whatever home or business you happen to be near to.
In addition to ISPs allowing their own subscribers access, there are examples of corporations cutting deals with ISPs for hotspot network access. For example, Google's MVNO Google Fi has a deal with unnamed partners that allows subscribers' phones to connect to hotspot wifi networks for extra coverage, branded "Wi-Fi Auto Connect+" [2].
From the user's perspective (and personal experience), this connection is handled seamlessly and outside of the OS' normal wifi UI flow. If your phone sees no saved wifi networks but does see xfinitywifi (et al), it auto-connects in the background. Authentication with the captive portal happens automatically and non-interactively, presumably with some keys provisioned to your device. Your traffic is VPNed back to Google, so the router and ISP don't see anything. The only indication that any of this happened is that the "5G" icon changes to "W+"; the normal WiFi icon never shows up.
In the case of Google Fi, this is actually a pretty great deal for users. You get better coverage (especially indoors in cities, where cell service can be spotty) with no real downsides to you – your traffic isn't meaningfully exposed to another third party, it doesn't cost you extra, and you don't have to bother the staff for a wifi password.
But given all that, it's not a huge leap to believe that ISPs are also more than willing to quietly take LG's money in exchange for an all-access pass to their hotspot network.
It's easy to imagine that an evil company could ship their TV with a key that allows it on an ISP's hotspot network. No extra suspicious hardware like a 5G modem needed, and no MAC address spoofing required. The TV software could easily connect to the hotspot network with zero indication in the UI, and then use the surreptitious connection only to transmit your kompromat back to HQ.
If done intelligently, you'd never notice. By only transmitting spy reports (and not downloading new ads), even a keen observer wouldn't notice any behavior on the TV that would trigger "how tf did this thing get a network connection?" Even more insidious, you couldn't really see what traffic was happening, since IP packet captures on your network are useless in this scenario. You'd need special hardware to capture what the TV is actually doing on the air.
If truly evil, the software could do this hotspot dance even if you've configured your own WiFi network on the TV. If the software finds it can't reach ad HQ because you've firewalled it off, then despite your best efforts to contain the disease, it still can spy effectively thanks to your neighbor's router with its default-enabled ISP hotspot. Just do a daily upload while you're sleeping and otherwise sit innocuously on your locked-down VLAN.
Everyone evil wins: ISP collects that sweet bonus revenue at zero marginal cost, TV manufacturer doesn't have to foot the hardware bill for millions of 5G modems or (relatively) expensive cellular agreements, and advertisers get to be that much more creepy targeting you. I'm sure the wanna-be despots of the world don't mind the spy apparatus being built for them either, conveniently under the control of easily-compelled corporations.
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Now to be clear, I have no proof that any TV manufacturer is surreptitiously connecting to an ISP's hotspot network in order to exfiltrate your data. But all of the building blocks to make that happen provably do exist, and I seriously doubt that capitalism will allow them to go unused.
It's anything but far-fetched.
[1] eg https://www.xfinity.com/support/articles/xfinity-wifi-hotspo... and https://www.spectrum.net/support/internet/spectrum-mobile-wi... and https://www.optimum.net/pages/internet/hotspots/faq.html
[2] https://fi.google.com/about/wi-fi-auto-connect-plus and https://support.google.com/fi/answer/10091529?hl=en