Unfortunately, the article doesn't give any reasons as to why somebody might choose to opt-out of the network except for bad analogies:
> If suddenly my devices are going to start connecting to my neighbor's WiFi, or theirs to mine, it seems like you'd have to opt-in, right?
I can already connect to my neighbor's wifi, and they can connect to mine: Our ISP broadcasts a separate "public" SSID, which is routed over a separate IP address and available to all of their customers via a captive portal.
It also has its own bandwidth allotment (unlike Sidewalk, but bandwidth use there is probably very low).