While I completely agree that AD&D was both a rather complete system (especially for the time, and considering what the systems were addressing at that point in time) and a toolkit (so many options especially in the DMG, but even the PH), I think there was much more than OD&D in there, but a huge amount of production from Gygax and a few others. The amount of material in AD&D is enormous, just take as a simple (imperfect, but still telling) metric:
- AD&D Core (PH + DMG + MM) = 130 + 240 + 114 = 494 pages (A4)
- It's very hard to find the equivalent with OD&D, my version of Men & Magic has 114 pages, but they are extremely small, I doubt that they contain 25% of a AD&D PH page.
- Holmes: 49 pages
- Moldway: 68 pages
- Even adding all the pages of all BECMI books together, I doubt that you get to more than half the core AD&D published in a very short time.
Admittedly, it's not all of fantastic quality, especially the editing, but the amount of material is enormous (just look at spells and monsters list, plus all the races and classes, etc.).
Obviously, everything depends on how you look at it. I don't recall, inter alia, the random government table in OD&D. And none of this is to slight how awesome AD&D was, and remains (for its faults, the 1e DMG remains a tour deforce).
That said... if you look back at the LBBs, the supplements, and the prior Dragon Articles and Strategic Review Articles, you will find, pretty much, every single thing that's in AD&D.
All the classes. Assassins, Clerics, Druids, Fighters, Magic Users, Monks, Paladins, and the Thief first appeared in OD&D (LBBs or Supplements). Rangers, Illusionists and Bards were in the Strategic Review.
Magic Items and Artifacts (not all but many) were in the supplements. Psionics were previously covered. So many of the spells were already listed. By the Greyhawk supplement, we already had 9th level spells.
Again, this isn't to slight AD&D's brilliance- but to point out that if you cobbled together OD&D, with all of the supplements and a bunch of magazine articles, you were pretty close to AD&D.