I here you. My thoughts are that in these studies they control for total work, in such cases pH is not as low and lactate is not as high. However if you are able to get 2-3 moe reps then perhaps in the end there will be no difference between supplemented and non supplemented groups.
In terms of where GH is stimulated, while we are not positive on all the metabolites involved we think that there is a direct neural feedback mechanism in the group 3 and 4 affarent fibers sensitive to metabolite buildup (H+. K+, lactate) and also in the blood itself as adding a bi carb buffer in studies has blunted the GH response to training. In the blood it is thought that GHRH binds optimally to the anterior pituitary under more acidic conditions