Strategy guides were a big deal before the internet came along, giving players hints, tips and tactics which otherwise wouldn't be readily available.
One of the unexpected perks of writing these books was the ability to add depth and detail to the game's lore, as is evidenced by a character profile spotted in Tekken 2's Japanese guide by @gosokkyu:
That's not all, though, as gosokkyu adds:
This same guide has all sorts of weird stuff, like Kazuya apparently being the latest reincarnation of some old shogunate hanger-on who died holed up in a castle at Hokkaido during one of the last stands at the end of the Edo ero
Bruce Irvin's backstory is referenced on his page on the Tekken Wiki, along with a second version which drops the reference to cannibalism. It would seem that Namco removed this little nugget of info following Tekken 2's release.