generally speaking, if you have any sort of protection in the mod you always want to disconnect the positive batt terminal (this is what a kick will do, the hot spring of a reo, etc...). You are right that in this simple of a circuit that electrically it really doesn't matter, it just comes down to best practice. Lets say you have a short and the fuse disconnects the neg, you would still have the positive running though the mod and through the coil so if you were to touch the ground (-) plane that was connected to the - batt terminal and then touch the coil, you would complete the circuit, and get a little shock. This scenario is not likely but just an example of why you would disconnect the + instead of the -
There may be other mechanical reasons why pdib did this, I can't tell from the pics.
Edit: That example is hard to see/justify with these simple mods, you kinda have to squint, turn your head, and use a little imagination. I work with large industrial control cabinets that use metal enclosures that are grounded (aka large ground plane) so if the - was opened in a safety condition and i touched the large ground plane then anywhere in the panel "I" would complete the circuit. And trust me no one wants 4160V going through them.