“Couldn’t conceivably know” is a very strong claim…

True. But isn’t that “couldn’t conceivably know” part of Wallace’s own philosophical grounding, in the sense that the gulf between self and other prevents us from ever really “knowing” what someone else means? Or even, perhaps, what we ourselves mean. This doesn’t mean we shouldn’t try to forge a connection with those with whom we’re in communication, or that we shouldn’t try to understand what’s actually going on in a text, but that attempting to answer the question of what the author meant is (a) futile, precisely because not only can we not ever know, but he likely didn’t actually know himself, or not in any way that could be communicated other than the way it appears in the text, and (b) beside the point, as what’s actually in front of us and under study is the text and not the author, whatever the class might be called.

But beyond that, in this case, there’s a much more literal “couldn’t conceivably know,” which is simply that he wasn’t there, and wouldn’t in any case have been there, to ask…