So 'twas ever thus?
I think you have to look at this in a slightly wider context.
The article of the ANO mandating aerodrome operating minima for non-PT flights was introduced in about 1992. Before that, an IR holder on a non-PT flight was not bound by any particular minima. Thus everything in the AIP was advisory. AOC-holders had minima dictated by their Ops Manual (which was undoubtedly based on the same calculations as in the AIP).
When the guidance to IMC-holders was drafted, it was done with that as a backdrop. I don't think that anyone foresaw that it would be interpreted as mandatory at some point in the future. The Schedule 8 minimum of 1 nm for t/o and ldg was always there.
I can guess at why it has been left the way it is. The (mis-)interpretation of the AIP as mandating 500/600 ft minima suits the CAA position of discouraging IMC-rating holders from tackling difficult approaches. So clarifying the AIP entry as being recommended only is unhelpful for that cause. But changing the entry to indicate that the increments are mandatory would be a substantial erosion of rights that would require consultation, time and resource. So as with so many things in aviation, we're left with the status quo. But that's just a guess.
As is so often the case, a simple concept is overcomplicated by poor drafting. Something the FAA by and large manages to get right.
I think that\'s partly because the FAA gets regulatory stuff into the FARs and advisory stuff into the AIM. None of these notifications as we have.