Whispernet was a whole different thing, and it dates to the very first Kindle.
This Kindle did not have things like idle-screen advertising. That wasn't yet a thing yet.
These first edition devices were available with unlimited data access (IIRC in the US via AT&T) on cellular networks without a separate subscription. It was slow (everything was slow back then), but it would let a person download a book or have a look at a web page (with the very limited browsing that was possible with e-ink and a CPU that was meant more to barely sip power than to render megabytes of CSS and JS).
The expense of the data access was built into the one-time purchase price, and the hope was that people having the ability to buy books from "anywhere" would snowball into a thing that was both very popular and profitable.
It was simple and, functionally at least, it worked very neatly: Take new Kindle out of the box, switch it on, and download a book with it. No wifi or PC connection or other tomfoolery needed.
That was back in 2007 -- a time when many people still had landlines at home if they wanted to make a phone call, or a dumb phone in their pocket if they wanted to do that on-the-go. Some folks had Blackberries or connected Palm devices, but those things were rare.
And the Internet, and indeed Amazon itself, was a very different place back then. Having an Internet connection that was very quietly always available on a Whispernet-equipped Kindle was pretty cool at that time.
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Sidewalk is a different kind of network. It uses consumer devices (like Echo Dot speakers) to act as Sidewalk bridges. This generally works at a low frequency (900MHz-ish), to provide a bit of relatively slow, relatively long-range wireless network access for things that are otherwise lacking it.
The present-day operation works like this: Suppose I've got some Amazon Echo speakers scattered around my house. If a neighbor's Internet connection is on the fritz, then their Ring doorbell can use a tiny slice of my Internet bandwidth using Sidewalk via one of my Echo speakers to keep itself connected to the network and thereby still function as a doorbell.
Or, maybe their Ring doorbell is out on a post by the gate, where their wifi coverage sucks. If it can gather up a little slice of 900MHz Internet access from anyone's near-enough Sidewalk bridge, then they've still got a button for their gate that notifies them on their pocket supercomputer when some visitor is waiting out there. They don't even necessarily need to plan it this way in order for it to Just Work.
Or, what GP was referring to: Your hypothetical new smart TV might use the neighbors' Sidewalk-enabled device(s) to update or patch itself, produce new ads to show you, and/or send telemetry back home to Mother. It might do this even without you ever having deliberately connected it to any network at all.
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Either thing (some modern equivalent to Whispernet, or the already-loose-in-the-wild Sidewalk system) could potentially be utilized by smart TVs and other devices to get access to the network and simply sidestep the oft-repeated, well-intended, and somewhat naive mantra of "It can't have Internet access if you never connect it!"