No, it’s not that. The point is not that using a sub domain is easy or not, you might not have access to using one or maybe your setup is just “ugly” using one or you just don’t want to use one.
Its standard practice in all web based software to allow base URLs. Maybe the choice of framework wasn’t the best one from this point of view.
As for docker, deploying immich on bare metal should be fairly easy, if they provided binaries to download. The complex part is to build it not deploy.
But you gave me an idea: get the binaries from the docker image… Maybe I will try.
Once you have the bins, deploying will be simple. Updating instead will be more complex due to having to download a new docker image and extract again each time.