"AFTERWORD
It is the spirit of the game, not the letter of the rules, which is important. Never hold to the letter written, nor allow some barracks room lawyer to force quotations from the rule book upon you, if it goes against the obvious intent of the game. As you hew the line with respect to conformity to major systems and uniformity of play in general, also be certain the game is mastered by you and not by your players. Within the broad parameters given in the Advanced Dungeons & Dragons volumes, you are creator and final arbiter. By ordering things as they should be, the game as a whole first, your campaign next, and your participants thereafter, You will be playing Advanced Dungeons & Dragons as it was meant to be. May you find as much pleasure in so doing as the rest of us do!"
Dungeon Master's Guide, First Edition, p. 230
Related: this video is a reaction to an X post by Jeffro Johnson.
"These People are Accusing Gygax of Being a Lying Grifter" Kasimir Urbanski - RPGPundit https://www.youtube.com/watch?v=r7uPip720Fo
OK, that's a whole blog post of a rant there, and not at all relevant. OD&D was a mess, but it's unambiguously freeform, you are repeatedly told to make the game yours, not that you need permission.
OD&D got a good rewrite, by Eric Holmes and later Tom Moldvay. Both are blatant about freeform play.
The one place it looks anything like "rule zero" in AD&D is that afterword, where he tells you to consider: 1. The game as a whole meaning tournaments and West Marches style cross-table compatibility, 2. Your campaign meaning don't fuck with the balance, 3. Filthy peasant players last.
Gary's strident editorials in Dragon were clear, you can't "barracks lawyer" as he put it, out of "you have no rights in AD&D".