I think you are missing the point here about dependency. If the dependency parameter has been set when registering or enqueueing a style or script, that style or script will wait in queue for the style or script it is dependent on to load. This particular script or style will not load until the script or style it is dependant on is fully loaded.
Now, if you deregister that script or style that others are dependent on, it is obvious that that particular script or style will never load, which in turn will mean that any script or style that is dependent on that particular style or script will always stay in queue and will never load. It is the same waiting for a bus to take you to your destination, but you will never be able to reach your destination with that particular bus as that specific bus has been cancelled indefinitely
I’m not sure why the original author has done that, but I really see no point in deregistering jquery as this breaks so many visual representation and in some cases, functionality of the site.