Clients on Orbi Wi-Fi receive an IPv6 default gateway from an Orbi unit

Yes, I can confirm the advice you are getting here on the link-local IPv6 address usage, GusTTavo​ . I have seen this in tcp dumps on my own network, where IPv6 is disabled on the router. To explain for you in more detail:

IPv6 link-local addresses are a normal part of networking and continue to work even when IPv6 is disabled on your router. Computers, phones, printers, and other devices on your network automatically create their own link-local IPv6 addresses, which typically begin with fe80::. These addresses allow devices to communicate directly with one another on the same local network without needing an Internet connection or IPv6 support from the router. They are used for basic networking tasks, such as finding other devices and exchanging information with them. Link-local addresses are limited to your local network and cannot be used to communicate directly with devices on the Internet.

When you disable IPv6 on your router, you are generally turning off its ability to provide IPv6 Internet connectivity and to help devices obtain IPv6 addresses for communicating beyond your local network. However, this does not turn off IPv6 on the individual computers, phones, and other devices connected to your network. Those devices can still use their link-local IPv6 addresses to communicate with one another. For example, two Macs on the same local network can communicate using their link-local IPv6 addresses even though IPv6 is disabled on the router.

The important point is that disabling IPv6 on your router does not disable IPv6 throughout your entire home network. Devices can still communicate locally using IPv6 without involving the router’s IPv6 routing functions to the outside world and back. Therefore, seeing IPv6 addresses beginning with fe80:: or IPv6 traffic when examining your network is perfectly normal, even if you have disabled IPv6 on your router. These addresses cannot be reached directly from the Internet and do not, by themselves, represent a security concern. In fact, it is generally best to leave IPv6 enabled on your computers and other devices because some operating system features and local networking services may depend on it, even when you use IPv4 for all your Internet traffic.