BTW, this hidden network probably uses another protocol (for the OneMesh). It is the 802.11s (https://en.wikipedia.org/wiki/IEEE_802.11s), that uses its own encryption method based on Simultaneous Authentication of Equals (SAE) (yeah, that is the same as WPA3, however it came before it). It shows as hidden network on Wi-Fi Analyzer, but the network is not actually hidden in the same sense of a hidden Wi-Fi network: this simple happens because 802.11s has no concept of SSID.
The authentication of new devices happens when you pair a new router using the application available on Android/iOS (it has a web interface too but AFAIK it doesn't allow adding new mesh routers to the network). So it seems pretty secure for me, at least sans some security bugs that I am sure that the device should have. Doesn't bother me too much considering that most bugs that I saw on those consumer routers generally comes from the security from things like administration pages and not the Wi-Fi network itself (unless it is something like KRACK that affects all devices implementing the protocol).
Yeah, it is still pretty sh*t that they enable this by default, but if the router from the author of blog post is from one of their lines of mesh routers I do think this is kinda of made by purpose, because using multiple routers devices is kinda of the idea of a mesh network.