I was not suggesting they were being sarcastic. I was suggesting they had not actually noticed that the error message actually answered their question: "Why can’t I shadow this function?".
Having also not spent that much quality time with the hyperspec, even if I've quite enjoyed dabbling in common lisp from time to time, it wouldn't've occurred to me that the spec would explain 'why' as well as 'what'.
"Underestimating the hyperspec" is a persistent mistake on my part sadly, although I at least seem to underestimate it less these days.
This is the section of the standard the error message points at: http://www.lispworks.com/documentation/lw71/CLHS/Body/11_aba...
And this is the reasoning for why that section exists, linked to from that page: http://www.lispworks.com/documentation/lw71/CLHS/Issues/iss2...