Hi Acinteyyo, (mikenz66)
Luang ta actually mentions earlier in the book "forest wisdom" (footnote 8, p16) meaning experience of the meditating monks, although not around this area, or chapter.
I am going to stick with my earlier thoughts, probably excluding what I had a sense of him meaning with Dhamma. It still strikes me as the use of dhamma here is significant in understanding his meaning.
He who knows only his own side of the case knows little of that. His reasons may be good, and no one may have been able to refute them.
But if he is equally unable to refute the reasons on the opposite side, if he does not so much as know what they are, he has no ground for preferring either opinion …
He must be able to hear them from persons who actually believe them … he must know them in their most plausible and persuasive form.