Hudson...
My point is that Vref + 20K (which is prob +15K on normal Vapp, but maybe nearer desired Vapp in gusty conditions) @ 500' is "as near as dammit stable".
The idea of the stabilised criteria is that you are configured, on profile, and have some power set in the latter stages for quick engine response if a GA is needed - which in turn means IAS not to far in excess of Vapp...
There has to be a "tolerance", and both the Boeing Manual you quote, and at BA, we use the +20K. It is designed to stop people continuing from, say, +40K, and neededing idle all the way down, and probably still fast...
That is completely different to saying the +20K is acceptable at the threshold. It is not, and we use +15K for a limiting runway.
If you are unable to manage a +15K (or less) reduction over the nearly 2 miles from 500', and are going to rigidly apply those criteria, I can only assume you are well practiced at Go Arounds
Any sort of gusty weather, and you will often see the IAS above Vref+20K for periods at or below 500'...
Final point, don't worry about whether you agree with me or not. That the Boeing Manual and BA and seemingly others use this criteria seem to indicate it is well established, and I will go for their judgement...