Texinfo ultimately gets the @ convention from Brian Reid's Scribe[1], as developed at Carnegie Mellon during the late 70s and commercialized by Unilogic[2,3] in the 80s. Coincidentally, there was a close derivative of Scribe called Mint[4], also developed at Carnegie Mellon in the early 80s for the PERQ (an early personal workstation competing in the category of things like the Sun-1 or Lisp Machines).
[1] https://bitsavers.org/pdf/cmu/scribe/Scribe_Introductory_Use... [2] https://bitsavers.org/pdf/unilogic/Scribe_Document_Productio... [3] https://bitsavers.org/pdf/unilogic/Scribe_Pocket_Reference.p... [4] http://www.bitsavers.org/pdf/cmu/spice/Users_Manual_for_Mint...