Yes, same way you have to specify each folder direction in unison, same way you have to add a folder in syncthing. Sure, there are more steps to do that in syncthing, but the fundamental idea that you have to do it per folder is the same.
You could say syncthings config.xml sort of decides this, which could also be done by modifying a single file and not using the UI.
Anyways, I think you should stop comparing syncthing to unison as it’s a tool with fundamentally different implementations, so it’s unfair to want for one to work like the other.