It's the relevance of setting such a narrow scope I question. OK, so the game was less voluminous before 5 supplements came out, but that was a period of barely a year in which only a few thousand copies were sold.
We're discussing unified mechanics. There's an assumption that pre-3e didn't have unified core mechanics. The game as originally published used only two core mechanics. Not bad for the very first time someone published an RPG.
Rules expansions and new editions came out and added more stuff. If you played a thief, your skills used d%, so you had one new mechanic to learn. It helps to make thieves feel different and stand out from the other classes. Rangers use a d8 for surprise instead of a d6, but it's the same basic concept, just 2 chances in 8 instead of 2 chances in 6. There are other things that got added piecemeal until the big reboot/revision hit with 3e. Everything from OD&D to 3e can be looked at as 25 years worth of patches to the same core system. Still, the core system - OD&D - only had two core mechanics when it first released.
Good points. Much as I loved playing it back in the day, 1e AD&D was very complicated, and looking back at it, so much of that complexity seems entirely needless. But, considering it was an immediate successor to, 0D&D a game of a type that hadn't ever quite been designed before, puts it in context.
Doesn't make it any less complex, but puts it in context.
AD&D 1e is essentially OD&D with 10 years worth of supplements and magazine articles baked into it. It wasn't a redesign from the ground up like 3e was. The thing with AD&D is that a lot of the content is implied as optional, but it's not always explicitly marked as optional. If you use all the bells and whistles like weapon vs armor modifiers, non-weapon proficiencies, etc. I'm sure the game would seem complicated. I wouldn't consider any of that stuff as "core mechanics" though.
Even in AD&D 1e the core mechanic is mostly roll a d20 plus modifiers vs a target number found in a chart listed by class & level. Attack rolls, saving throws, and turn undead all work that way. The "number of chances in six" mechanic was still there for surprise, secret doors, etc. Other mechanics got into the system via updates (expansions/articles) over the years.
I can't find a definition of these terms that makes sense in context, just something about 19th century Prussians.
It sounds like you're trying to say "stop reading what the rules actually say and you'll see how simple they are."
There's more info out on the internet, but to try to summarize in a nutshell:
The German/Prussian army back in the 1800s used miniature wargames as a tool to train officers. They called it kriegsspiel ("wargame" in German). At first, the games were run according to strict rules (aka "strict" kriegsspiel). Some officers found this to be unsatisfactory since it caused the game to play out much more slowly than a real battle and allowed players to game the system by exploiting rules loopholes.
A general came up with a novel idea: have an experienced officer referee the game and use his judgement to run the game. "Free" kriegsspiel was the result.
Let's use an example of advancing your soldiers towards the enemy line. According to the rules, your soldiers move 6" per turn.
Strict: You measure out 6" and advance your soldiers exactly that much.
Free: You tell the referee that you give the unit commander the order to advance. The referee eyeballs the map and moves your soldiers forward what he thinks is about 6".
The free kriegsspiel model not only plays faster, but better simulates the chaos and uncertainty of battle. Orders might get misinterpreted, soldiers might move faster or slower than you expect, etc. It also allows players to stop thinking about the rules of the game and focus entirely upon strategy and tactics: exploiting terrain, flanking actions, etc. Ideally, the players wouldn't need to know the rules at all. All of the rule-handling is done by the referee and he's free to use his judgement instead of the letter of the rulebook if he wishes.
There are better explanations out on the internet. "Strict" and "free" are two different methods of playing miniatures wargames. They both have strengths and weaknesses.
Player accounts indicate that Gygax and Arneson used the "free" model when running D&D. Players weren't supposed to worry about the rules, they just told the DM what they wanted their character to do. The DM handled all of the rules and exercised judgement when necessary.
Intent can be hard to divine. When judging rather a set of rules is complex or simple, I'd rather judge the rules, themselves. Early D&D is often credited with being simple, because the books are comparatively small, pamphlets, really, some of 'em. But dealing with an incomplete system can be plenty complicated, in itself. Likewise, punting to DM judgement does not create simplicity.
Intent doesn't need to be divined in this case. We have accounts from players who played with Gygax and Arneson describing how they ran the game. It's not a huge leap to assume they ran the game the way they intended it to be run.