@tszaboo: Your neighbors Amazon Device will not be able to directly connect to your WiFi. Without a big undiscovered security flaw (or bad configuration of your WiFi), that would simply be impossible without giving the neigbor your WiFi password.
What will happen will be similar to the following:
Your Amazon Device is of course connected to your WiFi. It needs that to work. To be more specific: It needs an always available connection to Amazon. Likely, it will use some kind of encrypted connection to establish this connection, and this connection will always be open. Some kind of VPN tunnel. How much traffic actually flows will be dependent on what the device is doing. While it is idle, there will not be much traffic, only keepalives (i am still here, are you still here?)
What the enabled Sidewalk will now do:
Your Sidewalk enabled device will send out a wireless signal. It can do that even without special hardware, by what is effectivly multiplexing it's own connection with the sidewalk network. That way there is no requirement for additional hardware. That is also already a likely reason for the relatively low bandwidth that Sidewalk provides: It must not affect the primary operation.
It is also very likely that any traffic coming in from the Sidewalk network will go directly into the established Amazon connection. At least that is how i would do that

With the info how it is advertised it is clear that you don't need to be afraid that your neighbor will be able to use his FireTV stick to stream 4K video using your internet connection.
That still does not make it better, but from a technical standpoint i find this quite intriguing. This can be made reasonably safe, but 100% safety does not exist. It is even understandable *why* the try this with opt-out: If it were opt-in, to many users would just be too lazy to enable it, not even starting with privacy issues. :p
But the way they are setting this up is bollocks. Apparently you can't even disable Sidewalk without installing an App.

Who knows what data that collects...
And considering metered home connections: I am always baffled when i hear that these are really a thing in the US. Even in germany, sometimes considered the home of crappy internet infrastructure, this has never taken on after the establishment of DSL. At least thats true for wired home connectiviy, dont't get me started on mobile internet
