Messages

I guess this route gets added when setting an IP on the VPN interface itself, which is not what the guide tells you to do (both IPv4 and IPv6 on None).

Looks to me that step 9 is for routing traffic from WG address to WG network via WG gateway. IMO it's unnecessary because such route will be auto created when we configure a tunnel gateway nowadays.
For example, this is an excerpt from my routing table:

That's not what I'm seeing, this is all WireGuard routing info when following the guide, including disabling routes and adding a gateway instead, which is needed for selective routing:

Code Select Expand

10.64.0.1          10.66.165.1        UGHS        wg1
10.66.165.1        link#19            UHS         wg1
10.66.165.89       link#19            UH          lo0

10.66.165.89/32 = interface address as supplied by external VPN provider
10.66.165.1 = IP of VPN gateway created through System - Gateways - Single, for selective routing purposes
10.64.0.1 = next hop for VPN connections for gateway monitoring purposes (the Monitor IP set on above gateway))

What did you do that caused the /31 route to appear?