The problem with this conversation is that the core of what
@terrapin is talking about are really maps of the contents and nature of the human mind, and that is being extrapolated to beliefs or statements about the physical or external world. You're saying there's no knowledge here, but there is - it's just not necessarily knowledge about the external world, its knowledge about what kinds of things can be found in your mind, and how to get to them. Spiritual practices involve specific repeatable processes that produce self reported similar results - this is about as good as we can ever hope for when it comes to any sort of science of the mind. You teach me a tool to use in investigating or training my mind, I use it and report back the results. If the results are consistent across large numbers of people, there's knowledge about the mind there - we may not know all the details about what that knowledge means, but there is still knowledge there. It is important to note that the 'results' alluded to above include not just reports of the contents of specific experiences, but also deeper levels of understanding the very nature of experience itself.
He may believe that this knowledge does in fact say things about the physical world or other externally existing systems, but that's not really the point. I'm speaking for him but what I think he is getting at is that whether or not you personally believe this knowledge corresponds physical world* is irrelevant. The knowledge exists and can be reliably found through various spiritual traditions. As a person who does these types of practices, I can say that regardless of what you believe, the deep exploration of spiritual practices or your mind is well worth it - what you find can radically change how you experience life for the better.
Edit : To clarify this statement above "You're saying there's no knowledge here", you may be purely referring to his statements implying knowledge of the physical world, the point of my post was just to chime in that there is in fact a valid form of knowledge here.