Yes. Unfortunately, the work of an inexperienced lawyer...who definitely has a "big heart", but is just too green with the whole process.
"Did they count on this old law never being found?"
I suspect they thought it wouldn't be located in a timely fashion.
"Why isnt it available online either at the official HI website or Cornells? Or, apparently, in any of the DC law school libraries?"
Great questions. IMO, it appears to be a relatively obscure legal book, that most would not normally pay too much attention to. Not too much call for that book, outside this issue. Many folks concerned with this issue have been searching for it, for some time now.
Its unavailability made me wonder if Sandy Berger was on the case!