Alzrius
The EN World kitten
With regard to the Dalluhn manuscript, it's absolutely not a secret; even beyond his blog, Peterson was talking about it in such diverse publications as the second issue of the now-defunct Gygax Magazine (pg. 20-21, 65).You can down load this stuff yourself man, its not that hard to find! Here for instance some of Dalluhn are reproduced The Dalluhn Manuscript: In Detail and On Display There's even a paper about it written by Peterson. https://img.4plebs.org/boards/tg/image/1454/51/1454516675578.pdf None of this stuff is some secret!
But as noted before, that's a far and away different cry from Arneson's original notes to Gary Gygax. As the PDF you linked to noted (p. 28), "Moreover, since the Manuscript shows clear signs of collaboration in typing and artwork, it could not be the work of a sole fan, but must instead be the work of a team."
Arneson's notes, by contrast, are the work of Arneson himself, presumably with little-to-no additions from the rest of the "Blackmoor Bunch" (as his gaming group is now called), though I'm sure they inspired Arneson to come up with new rules and responses to their activities. Contemporary (i.e. written) presentations of Blackmoor as it was being played before 1974, however, remain elusive, and so any presentation of the original notes that Dave sent to Gary would be a huge find for D&D historians, amateur and professional alike.