I agree on some points. He did say arsenic and not antimony. And some alloys do contain arsenic, which adds to hardness. It would be better if he specified what alloy he is using, along with bore size and boolit diameter
I do agree that the hardness isn't the problem. More likely boolit fit and possibly his lubing techniques. Many forget lubing plays a roll also in reducing leading.
For instance, you lube than size, than relube before loading the boolit into the charged case. How many forget to relube after sizing, or don't think it is needed?
Harder boolits, driven at the same velocity and pressure as a softer boolit, with poor bullet fit to groove, will give more leading. The softer boolit will tend to fill the grooves better, and the harder boolit, will tend to just skim over the lands and be shaved. JMO.