Reverse osmosis drain with a single basin sink when there is garbage disposal?
That video is confusing. The purpose of the drain saddle is to provide a channel for the RO system to drain excess water - this is pretty standard. The problem with the current drain configuration is that the disposal runs directly into the P trap. You actually want the drain saddle installed between the disposal and the P trap.
The proper setup for this is to rerun your 1.5 inch drain from the disposal running it horizontally far enough for you to attach the drain saddle and then into the P trap. You may be able to rotate the elbow coming out of the disposal to accomodate a horizontal run. If not you may have to rotate your disposal a few degrees to give you the room you'll need.
I'm not sure of the building codes in your area but I'm fairly certain that attaching it to the drain before the P trap would violate code.