AD&D, like all games, is covered under copyright.
HOWEVER, games are not treated the same as books and other works.
Rules of a game, including "stats" and other information required to play the game, are NOT protected by copyright. This is Copyright Law, and has nothing to do with a license which a work is published under.
Artwork is fully protected by copyright, as is any setting descriptions. In the context of the D&D franchise, the actual wording of any rulebooks, monster descriptions, game modules, and such are all protected by copyright, so you can't wholesale cut-and-paste things.
But, that does not extend to the various mechanics of the game. Armor Class, Hit Dice, etc. are all mechanics, and CANNOT be protected. That includes values assigned to monsters or characters or such. A company cannot also protect the particular layout of those statistics, if that layout is considered generic in nature. In the case we are talking about, a table listing the statistics is NOT protected, as it does nothing more than list those statistics, and contains no original, protected material of WotC.
In addition, uniquely created monsters, characters, etc. all have their name protected, but not their statistics. But generic names thereof cannot be protected. It is perfectly permissible to have a clone copy of a Drow and call them a Dark Elf (a generic name). One could not copy the description of the Drow from a Monster Manual, but the idea of a Dark Elf cannot be protected, nor can the statistics thereof.
It is explicitly permissible by Copyright Law to clone the rules to a game, which in the context of D&D includes a generic name for anything trademarked (thus, no Drow, and no Dungeon Master, but Dark Elf and Game Master), the mechanisms used to play the game -- including the terms used to reference them -- and all related numerical statistics associated with those components of the game.
All of this is in addition to any rights the OGL gives you. These rights CANNOT be restricted by the OGL, as they are basic Copyright Law rights, not license rights.
I've been writing D&D expansions and such for over 4 decades now (since the late 1980s), and this is what I've repeatedly been told by various Copyright lawyers.
In short, provided you don't use the text description of a particular monster (and instead write your own one, using the same concept of what the monster is), and you don't use a trademarked name for it, it is possible to "scrape copy" the statistic summary section of any Monster Manual or the like.
Here's an explicit parallel: the game of Monopoly.
When creating a clone of Monopoly, here's what you CAN do:
- Copy the basic layout of the board - a square with the properties laid out in a path around the edge.
- Arrange the properties as they currently are, WITH THE CURRENT NAMES on them. Each property's name is not possible to protect, as they are generic names.
- Free Parking, Jail, and Go To Jail, and Go can all be labeled and placed accordingly.
- Chance and Community Chest cards can be named as such, and referred to as such. The text on the cards is also (mostly) usable.
- The costs and values of all Chance, Community Chest, and properties can be copied.
- The rules of movement, going to jail, etc. can all be copied.
- Have pieces that are Hats, Dogs, Cars, etc, and both Hotels and Houses, and explicitly refer to them by those names.
Here's what you CANNOT do:
- Use the particular color scheme of the board
- Use any artwork, including the drawings on any cards.
- Use the particular graphic presentation of a card.
- Copy the design of any piece, except the House/Hotel which, insofar as they are very generic, can be extremely similar.
- You can only use the word "Monopoly" in the context of referring to the Parker Brothers game, not in any other context, as it is trademarked.
- The specific wording of the rulebook cannot be duplicated. You have to write the rules in your own words.
- The wording of certain Community Chest and Chance cards, where they are not just generic game instructions, cannot be duplicated. E.g. "Grand Opera Night—Collect $50 from every player for opening night seats" cannot be duplicated, but you can have any other wording for something that would gain you $50 per player.
Now, see how that works in comparison to D&D?