You can do this, but again, you need to use the command line. E.g. 'resolvectl dns tun0 8.8.8.8'
We're actually no longer debating how systemd-resolved works; rather, we're now debating how NetworkManager chooses to configure systemd-resolved. systemd-resolved just does what it's told to do. It's actually NetworkManager that decides to split DNS according to routing by default as a matter of policy. It could do otherwise if it wanted to, but I think this is a good default. Nothing stops you from changing it though. :)