I gather the intent of the rule is to standardise the techniques for determining landing distances. Prior to the standard slope manufacturers could use steep approaches to limit the horizontal distance, which resulted in one or two really heavy landings during the certification phase. Aside from the aircraft damage, it was unlikely that these distances would be realistic for operators. Using a normal ILS approach angle makes sense as a "common playing field" approach, even if the actual approach is to be made without any form of ILS
The "instrument" was probably not defined deliberately. This left the door open for new developments that were then on the horizon - including MLS.