Agreed, though I don’t think they disabled systemd-resolved, because it still works using 127.0.0.53 when they’re connected to the VPN. So the daemon must be running, unless Mullvad itself has a DNS forwarder using the same loopback. I suspect they either hard coded some upstream DNS server for Mullvad, because Mullvad might not have supported systemd-resolved yet. Or maybe they set a permission on the configs, and something changed with the user context of Mullvad processes.
And I think it’s probably not in resolv.conf, that’s a stub that kind of redirects things to systemd-resolved. So I think it’s in the forwarder config of that.
Be careful, I was just looking over the Arch docs I linked you to, and I think the configs have changed substantially in the last few months. There’s a good chance that the configs in Mint look substantially different.