Given the nature of the citations as well as their publication dates relative to the checklist, I would argue that the list made by Xerox is the original. The checklist was made by Deniese Pierotti.
Mar 14, 1994 - Making Computers People Literate
"Making Computers People-Literate is a computer cookbook that offers practical guidelines and techniques for identifying, diagnosing, and curing computer interface problems" - Google Books
To me, this sounds more like a critique of UI / UX in the early 90's than a book that would contain checklists for designers or developers. Furthermore, the author's background does not formally involve design or technology. Although I admit that I have not read this book either, I would assume it offers a layman's view of what made computers difficult for users to interact with at the time.
January 1, 1995 - 10 Usability Heuristics for User Interface Design
From 1994 to 1998, [Nielsen] was a Sun Microsystems Distinguished Engineer. He was hired to make heavy-duty enterprise software easier to use[.] Dr. Nielsen ended up spending most of his time at Sun defining the emerging field of web usability [...] and has invented several usability methods, including heuristic evaluation. - Wikipedia
Nielsen seems to be the sort of "founding father" of web usability seeing as he almost single-handedly set the standards at Sun Microsystems. 10 Usability Heuristics, though still not a checklist per se, offered a much more structured view of what makes something usable.
1995 - Heuristic Evaluation - A System Checklist
Xerox, a very large technology company, was developing new products with more advanced interfaces and began to target the web. Without any set standards for how user interaction should work, the interfaces were likely inconsistent between products and teams. The solution to this problem would be a checklist that designers and developers could run through to ensure their systems were user-friendly. I can't seem to find any additional information on how the list came to be or the author of the checklist, Deniese Pierotti.