The instructions on that page are outdated and they are in the process of being updated.
Did you enable Pi-hole to listen to all interfaces and permit all origins?
https://docs.pi-hole.net/guides/vpn/listening-behavior.png
Dis you change the push “dhcp-option DNS 192.168.100.6” to push “dhcp-option DNS 10.8.0.1”
On the connected client, can you run a nslookup google.com ?