Clearly it could be done with a track saw or other tool. ali is asking for tips on how to do it with a sander and has even commented the using the right tool is the best option. The door edge is an example of the type of sanding being done. That said, the tips on 'how to' using a sander could apply to many other similar tasks. Not sure how showing a technique that works is doing a disservice. I view it as an option that is good to know.
In the vein of the question then...
A disc sander has the advantage of keeping its table normal to to sanding disc, and therefore one only needs to push material in.
To sand the door's edge becomes driven by one's skill to keep the machine at the proper angle and also not scalloping in. So one either need very good hand skills, or some methods to keep the machine in the proper alignment.
Belt sanders have frames to to help prevent the scalloping in, and planes sometimes have side fences to aid in maintaining an angle... However I am unaware of anything for 1/2-sheet or random orbital sanders that maintain an angle.
If there was such a thing, then it would reduce the level of skill required.
I know I do not have the skills, as whenever I have tried similar stunts in the past, they were woefully unsuccessful. (I could get one edge straight, but be scalloped, or have it not be scalloped, but at an angle.)