However, I don't think even a complete understanding of the points of his book would be sufficient to understand shrdlu's code.
That sounds like a point against the use of Lisp. There's certainly quite a lot of handwritten case-specific parsing code:
https://github.com/stuartpb/shrdlu/blob/master/gramar
Perhaps the bit which looks most lispish is the dictionary: https://github.com/stuartpb/shrdlu/blob/master/dictio
(It also looks like this was written on one of those ancient systems that didn't support lowercase and only allowed 6-character filenames)
A limitation that also led to 'Schemer' dropping the R.