I interpreted him as saying (and it sounds like you may have as well?) that this distinction SHOULD NOT be ENCODED in XML or S-Expression or whatever data seria

I interpreted him as saying (and it sounds like you may have as well?) that this distinction SHOULD NOT be ENCODED in XML or S-Expression or whatever data serialization/markup syntax you invent. He does not appear to make an argument as to why he believes this, so I'd say his claim is at worst unsubstantiated. I find it hard to believe that there's a objectively correct decision about the number of semantic concepts that "should" be encoded in a syntax, but I can understand why lisp enthusiasts would be on the 'closer-to-none end of that spectrum. I'd be interested to hear why you think it's so essential. The only argument I'm really aware of is compatibility with existing markup languages. I would guess that a markup syntax that is as simple and regular (ie. syntactically consistent) as possible would be the easiest to operate on algorithmically. I would also guess that this would make it easier to build, on top of this language, tools which present the content in various ways that are easier to read/write/manually-manipulate if you so desired. Actually, his example syntax makes this argument implicitly. He is suggesting that his tools are significantly easier to implement by working with plain S-Expressions, and that they can easily be mapped to a 'write-friendly markup language that is isomorphic to XML anyway. I make no claims about the quality of his presentation or the clarity of his arguments.