SKR80 Guest Network Authentication

Just guessing from the Orbi Pro WiFi 6 AX6000 Tri-band WiFi System (SXK80) User Manual  p.111 ff -  Is it about this part explained in the fine manual where it's about the ability to add a local authentication password for accessing the Internet probably?

Yes, this could be configured as an open network, or the network SSID does require a WPA2-Personal shared secret to be accessed and encrypted. Alternate to define and share a WPA2-Personal or WPA3 Password (Network Key) which does ensure an encrypted connection (on top of the allowed network access) you can define OWE, permitting the wireless clients are supporting it:

OWE: For many public WiFi networks and WiFi hotspots, opportunistic wireless
encryption (OWE) might be a good choice because it offers CCM mode protocol
(CCMP) as encryption without authentication for access to the WiFi network.
Protected Management Frames (PMF) is automatically set to mandatory. (For
more information about PMF, see Protected Management Frames on page 201.)
The WiFi network can accept both clients that support OWE and clients that do
not. For clients that do not support OWE, traffic is not encrypted.