I consider "Game rules" to be the rule books, just for the record.
Sure, that's reasonable. I'm even more picky. I don't even consider all the text of the rule books to be the rules, but only a subset of it that is explicitly mechanical. The rest is just suggestions and advice.
But, whether or not you consider the published modules to be rules, the fact is that they profoundly influenced how people played the game - perhaps even more so than the actual rules, which many groups, despite having played for years had demonstrably never read and certainly never read fully. What most groups actually did was make characters, open a module and start playing according to the best of their understanding, and look up rules as needed. But what that really meant in practice is that many of them devised procedures on the basis of their understanding in play that weren't necessarily based on the 'rules' (and sometimes were in contradiction to them). But what they were based on was what was in the module.
The problem with assuming that the rules are what is written in the text of a rule book is that while this is technically true, it doesn't really clarify what's actually going on in play. In actual play by a group, there is no way to distinguish between these protorules and the rules because that's what the group is actually doing. 1e AD&D was marked by a high reliance on traps, but that doesn't necessarily come from the rules or the well defined examples and definitions of traps in the rules. What a trap was actually like was defined mostly by the modules. The DMG wasn't published until 1979, and I didn't have a copy until 1984. What I knew about the sort of things it covered during the period from 1982-1984 was largely defined by modules and to a lesser extent from my copy of the Expert Set (the 1981 blue box).
My definition of the rules is ultimately 'what is used in play'. I'd have to say that for me, from about 1982 to 1990, diplomacy per se didn't exist either mechanically or at the level of the metagame. Occasionally initial reaction rolls were made to decide if a monster was hostile or what prices a merchant were offered, but the idea of negotiation or persuasion really didn't exist. NPC's were on fixed sides - good guys and bad guys - and there wasn't much to negotiate over and I don't remember my groups doing so. Most players fit the model described in the DMG of attacking first and parlaying second (if ever), and the DMG told the DM that this should not be met with success. I can remember as a player with a new group attempting to parlay first and attack second, and it causing extreme confusion and consternation. Goblins were clearly on the 'bad guy' team by definition; one didn't parlay with them, you killed them and took their stuff. It wasn't until I got into college that diplomacy started to come to the fore as we started playing games where the number of teams multiplied and their interests and goals began to diversify. This culminated in a famous session (1994ish?) where we RP'd a political summit for 8 hours with no dice throwing.
It was intense and fun, and the players had a blast, but even then, because we didn't have a diplomacy mechanic I think that in truth we were actually playing out the details of something that I as a DM had more or less foreordained. I knew the motives of all the players, and was basically revealing to the players through play what those motives where and what side they'd ultimately end up on. Neither a metagame where players were attempting to influence me, nor a game where the dice influenced perhaps by the roleplay was really going on. If I had thought to run this scenario as a challenge, a notion that I didn't have at the time, I would have probably considered each of the NPCs a potential retainer of the PC and used his charisma score and the loyalty modifiers in the DMG accordingly. But I didn't really have a framework for thinking about it that way and I think correctly would have perceived that that approach was on some level potentially unconvincing.
Just as feints didn't happen in my games during the period because there were no tools for handling them, so I'm not sure diplomacy per se actually occurred in my games. I'd be interested to hear from other (older) players as to their experiences with how diplomacy was handled. But things that I had examples for in the modules did happen in my game (and regularly), and in that sense they were as much rules for me as anything else and it never would have occurred to me at the time that they weren't part of the rules.