In this situation, yes.
You could indeed just use the Jenkinsfile for building a project. You can also specify the DSL config directly in a job, e.g. in a separate deployment job.
What would be the right place to define the deployment config?
In the job itself is fine... unless you want to version control it. This is a historical Jenkins weak-point. The plugins that exist to try to solve the problem are clumsy.
This Jenkinsfile is their solution to the problem. I'm saying it's not a very well thought-out solution, because now they're forcing us to put this thing that's not part of the project in the project, if we want it version-controlled.
Come on now, we don't need to go to these contortions just to avoid saying that Jenkins is doing it wrong.