It is that simple. Make the dns entry point to your vpn subnet 10.10.100.X. The way it works is anyone not on your vpn won’t be able to resolve the ip address and will get an error. Anyone on the vpn will be able to resolve the ip address and connect via the vpn connection.
The part people are talking about that is likely confusing you is that if your service is already available via your actual ip address 1.2.3.4 then you have a security concern since anyone can access 1.2.3.4 even without your domain name pointing there. They are encouraging you to make sure your 1.2.3.4 network doesn’t allow access but updating your firewall settings to make sure it blocks connections that are not made via your vpn subnet of 10.10.100.X