You continue to concentrate on that one paragraph that shows how you can "prove" anything you want - but you totally ignore the actual data.
The charts and table clearly show a doubling of the strength of the product as captured by the authorities (the average product available in England) over a recent 10 year period.
Of course, back in the time of my experiences, the pot industry was at a much earlier stage and certainly on a much steeper portion of the cost improvement curve. The rate of improvement over time was probably greater, and my samplings were over a longer period.