I thought Jon and I are saying basically the same thing.
Adding HCl to the activated solution doesn't interfere with the ClO2 gas, and adding HCl to an activated solution doesn't seem to produce more measurable ClO2 gas in the solution.
I am not sure how Jim Humble measured the 1% change, but I think he was just talking figuratively. At any rate, we need to keep in mind that he is an inventor and not a scientist...