Yes, the fee to use outside entertainment is true. Disney usually has associated costs with a DJ, and of course, it is their preference that you use one of their "Disney Approved DJs." All of these DJs are experienced with the "Disney Way" of doing things, and the DJs have worked very closely with the Wedding Coordinators in the past, making it a smooth operation for all involved.
If you use an outside vendor, Disney still must set up a dance floor, lighting, etc, so that is how they recover their costs.
Hope this helps!