Is this, like, the grand Blizzard conspiracy theory, or does anyone have proof of this?
That is completely a bunch of baloney. Someone from Blizz was quoted as saying that, pre-release, when they were setting drop rates they took the fact that the AH existed into account, that there was an additional place for people to get items from. That's it. Despite all the crap you read on here, there is no ongoing dynamic calculation that adjusts drop rates based on how many items are in the AH. Similar to the crackpot Loot Server theory, but even less believable.
I don't think the AH is taken into account in real-time when a legendary (or any other item) drops. I have observed that loot does drop in ways that reeks of some sort of human effort (or possibly mistake) to...well to do what I'm not sure.
I typically play on weekends and late evenings. I have seen high frequencies of the same legendary or set item over the duration of a weekend. 5 months into the game I'd never even seen a Chantodo's wand drop, then suddenly over one weekend I found two of them. I've had weekend sessions where legendary belts were dropping like candy. I'd never found a Windforce until a week and a half ago -- since then I've found three.
I've also been in games where the same legendary has dropped moments apart for me. Or it drops for me and then a moment later for someone else in the game.
Given the rarity and number of possible legendaries these types of events should be much more infrequent than what I observe. Basically I've observed both expected and unexpected behavior in terms of legendary and set drops. Expected behavior is you do not get very many of them and there is a high variance in the ones you do find. Unexpected behavior is seeing the trends I've described above. "RNG is RNG" allows for the unexpected behavior to occur, but the frequency with which it occurs is what sends my alarm bells ringing.
Now I'll repeat what I said that I do not think AH is taken into consideration in real time when items drop as this would be computationally expensive. But it is feasible that some sort of scheduled program runs every 15, 30, 120 or whatever minutes and makes minor adjustments to loot tables that are used in the real-time calculations of what will drop.
Also consider that the following note appears in the 1.0.7 notes, which indicates there are bug(s) in how legendary items are dropped:
"The Incense Torch of the Grand Temple and Neanderthal can now drop"
Basically my intuition as a developer and the trends I've seen regarding legendary / set drops leads me to think that something is fishy.
(edit: Reworded last sentence to make sense)