I think the low mileage greasing interval is all down to design. It's OLD. The bushes and king pin bearing areas are very small and when new and tight there's very little spare room for extra grease. This means it quickly gets squidged out of the way and metal to metal contact starts. It probably gets lubricated better after a little wear takes place and movement in the joint allows the grease to move around too. With little grease, the king pin wear can be rapid, hence the 750 mile interval. Depending on how tough vehicle inspection is where you are, this wear can flag up a fail ticket unless you grease them up with heavy grease before the inspection. If not too bad and the inspectors pry bar not too long it will probably pass! If you can't feel any excess play in these joints and it all moves smoothly I'd not worry about it too much. You'll certainly be able to feel and see wear in the king pins when it starts to get bad if you try and lever it by holding the top and bottom of the wheel. Drive and be happy, at 1500 mles a year there lots of life left in them.
Happy New Year!