"Difficult to establish" refers to a current state based on current testing methods. The entire purpse of that site is to discuss testing in raod-side stops, etc., not the theoretical limitations on what can be tested.
If I have not been clear before, let me try to make this crystal clear: you ned to provide evidence that the THC level (not THC metabolite level) of a light/non-smoker is lower when the smoker describes themself as feeling high that the THC level of a heavy smoker when they are not high. This is different from saying the residual THC level of a heavy smoker, after the high, is higher than the residual level of the non-smoker. That's why evidence of the latter is insufficient to make your case. This is also different from saying that the current test methods are not geared to detecting actual intoxicatory levels. Further, it's a very straightforward notion.
For example, look at 2.10 and 3.2 on
https://www.idmu.co.uk/pdfs/drugtest.pdf to see the beginnings of such an approach. I don't know if a test can be developed that is as reliable as a blood-alcohol test (which itself is not precise) or not, but I see no evidence that it connot.
You have the goal posts, very specifcally laid out. I won't move them. Can you cross them?