You are probably right, DFC, but the 1800m min for the IMC pilot is reported met visibility, IIRC. The pilot himself has no way to tell if vis is 1000, 1500 or 2000m.
This figure is unlikely to change from say 2000m to 1500m as he descends down the approach.
It might do, but would the new value be reported to the pilot on his way down?
Also there won't usually be a reported vis at a non-ATC field. This brings us to the subject of DIY instrument approaches (which are legal)...........
So I don't think this is a practically enforceable detail.