I think you're at least partially missing the point.
For autopilots servo'd to pressure altitude, holding altitude to within 0.02 kPa is more difficult than holding altitude to within 0.05 kPa or to within 0.30 kPa (which is roughly the private pilot checkride standard as-tested).
Modern autopilots are actually better at holding altitude to a very tight tolerance than ancient, analog autopilots. Both can hold standards well within the PPL ACS.
> holding altitude to within 0.02 kPa is more difficult than holding altitude to within 0.05 kPa or to within 0.30 kPa
"more" difficult is obviously true, but the difficulty of holding an altitude is only a small part of the overall difficulty of RSVM.
In other words, RSVM is much more about accuracy than precision, and the claim was that planes were "probably fairly precise already". The reason they needed upgrades was to improve the accuracy, not so much to improve the precision.
You’re talking about getting different aircraft to agree between each other.
The post upthread expressed surprise at an aircraft maintaining a steady altitude to within tens of feet. That’s been a thing for many decades.