The guy i bought my key from on Reddit said he got the codes from his friend who owns a computer shop so i don't think they all came from the amazon glitch.
		
		
	 
Generally speaking whenever these GPU deals come up, you can always find these codes on the resale market. There are always a few guys who legitimately do work at a computer shop or something of that nature and are able to get them.
The difference here is that there were literally 100s of them and they all appeared at the same time. While there is no doubt that there were/are legit sellers out there the chance of there being more than a couple this soon after the promo began (this past Tuesday) is pretty slim, especially the ones who were selling high #s of copies. It takes a while for these to proliferate normally, typically 1-2 weeks after the promo starts beyond a small # of people who will get them right when the promo starts from upgrading their GPUs and not wanting the game and prices usually reflect the rarity early on and its typically singular copies until later in the promo when the resellers are stocked. I even asked one guy Ive bought from multiple times yesterday and he has not even gotten 1 GW4 yet. Its been the same since Splinter Cell Blacklist (which is the furthest back I can remember getting a GPU pack in game on the resale market). It usually takes roughly a week for them to start trickling in and then by the 2 week mark, there'd be a small handful of people who had multiples and prices start to go down due to competition. For some of them (Arkham Knight and Pick Your Path) by the time we got to the 1-2 month of promo start, it was possible to get them for $10-$15 and by the time the game in the promo was actually released they were even cheaper (I got uPlay Watch Dogs 3 days after release for $6 via buying a nvidia promo on resale). Add to that is that many times over the past 2 years or so, there have been glitches (espc on Newegg) that have allowed these promos to be sold for literally 1C and in some cases they legitimately sold them for cheap to get rid of em.
 I've gotten just about every GPU pack in game since Blacklist (every one thats been on Steam or uPlay) and its pretty much always followed this pattern. Sources vary but I regularly saw the same people selling them either on Reddit SGS or Steamgifts or Id get them from Newegg on glitch. Only one time the pattern above Ive not seen happen and that was MGS Phantom Pain, where the price actually went up on the resale market due to demand but the same pattern in terms of them appearing in the first place did happen.
The chance that your reddit guy is being truthful about his source is slim, and Im basing this simply on the history of how these things have proliferated before. If he had many multiple copies, Id bet on him being a legit source at 1%. It takes a while for shops to amass  enough of the promo to sell multiples as they have to receive the stock. At the same time, until games start being revoked there's nothing to worry about. Hopefully the game is out before anything of that nature happens. 
Its really going to be interesting to see how Nvidia and MS handle this. They'll likely base it on how many activated vs how many codes they were able to wipe out by putting the proof of purchase caveat in place. I think its 50/50 that they revoke, simply because theres really not enough info to make an educated guess at this point.