You're also quoting an edition that's out of date, which really makes your argument invalid. I never saud it doesn't say it, just it's wrong.The githzerai are a race of inhabitants of limbo, co-existing with the slaadi
That's the introduction to the Fiend Folio description of the Githerzai, p. 45. That's as canonical as you can get, since that's the official introduction.
So, you can take that word choice up with the official powers that be.