I wonder how this group of 'non-buddhists' would respond to Ven. Nyanavira's passage on Atakkavicara, seemingly they would have read it?
Sometimes translated as 'unattainable by reasoning' or 'not accessible to doubt'. But the Cartesian cogito ergo sum is also, in a sense, inaccessible to doubt; for I cannot doubt my existence without tacitly assuming it. This merely shows, however, that one cannot get beyond the cogito by doubting it. And the Dhamma is beyond the cogito. The cogito, then, can be reached by doubt—one doubts and doubts until one finds what one cannot doubt, what is inaccessible to doubt, namely the cogito. But the Dhamma cannot be reached in this way. Thus the Dhamma, though certainly inaccessible to doubt, is more than that; it is altogether beyond the sphere of doubt. The rationalist, however, does not even reach the inadequate cogito, or if he does reach it[a] he overshoots the mark (atidhāvati—Itivuttaka II,ii,12 <Iti. 43>); for he starts from the axiom that everything can be doubted (including, of course, the cogito). Cf. also Majjhima xi,2 <M.ii,232-3> & i,2 <M.i,8>.
[a] When he is being professional, the rationalist will not allow that what is inaccessible to doubt is even intelligible, and he does not permit himself to consider the cogito; but in his unprofessional moments, when the personal problem becomes insistent, he exorcizes the cogito by supposing that it is a rational proposition, which enables him to doubt it, and then to deny it. 'Les positivistes ne font qu'exorciser le spectre de l'Absolu, qui reparaît cependant toujours et vient les troubler dans leur repos.' --- J. Grenier, op. cit., p. 44. ('The positivists do nothing but exorcize the spectre of the Absolute, which however always reappears and comes to trouble them in their sleep.') For Grenier, the Absolute is not (as with Bradley) the totality of experiences, but is to be reached at the very heart of personality by a thought transcending the relativity of all things, perceiving therein a void (pp. 100-1). Precisely—and what, ultimately, is this Absolute but avijjā, self-dependent and without first beginning? And what, therefore, does the Buddha teach but that this Absolute is not absolute, that it can be brought to an end?
But perhaps (through ignorance of their position) I am misinterpreting them.