Why is there only a client config? This would mean when on a server, the client would be able to override what the server has set in its own client config.

Please advise. Using this mod in a modpack which would also be used on servers and controlled by a client config is not a good idea as a player could edit the client config themselves and basically re-enable / reset anything overriding the server.

If a SP instance modifies things in their config, that will apply when they play in SP.
The server decides what happens.