Have you heard of Amazon Sidewalk (https://sidewalk.amazon/)? All those Rings and Alexas in your neighborhood are able to serve as network proxies for random IoT devices.
LG very likely has your neighbor's WiFi password, so even if you don't connect it to your own network, they could theoretically just try and connect to any known network nearby. No 5g modem needed.
Idk if they do that, but it doesn't seem complicated to implement, and it doesn't seem to fall outside of the company's ethical boundaries.
I think enough customers freely connect their devices to the internet that they don't need to invest in the engineering required to include those kinds of things. I agree they won't have ethical qualms about it. It would also be pretty easy to spot; I feel confident saying they'd be too greedy to be sufficiently sneaky to keep it hidden.
For now, it remains easy and sufficient to not give them your wifi credentials.