gonzalo This website generates random subdomains for which they control DNS, then asks the browser to resolve those queries and check from which IP those queries come from.
The website can't see the internal IP of the DNS resolver provided by the VPN through this tecnique, but can see the public IP of the resolver.
For example if your VPN is configured to use 10.20.30.40 as DNS resolver, and 10.20.30.40 is using 192.0.2.1 to actually resolve (which can be the VPN itself or a third party) the website will see 192.0.2.1.