This conspiracy theory missed something important: competition. There are other companies with their own treatments either one the market or in development trying to take away your customers. If you can find a better treatment and roll it out you get all that money, if you don't you risk someone else finding it first and taking away all your sales anyway.
The more effective treatment will shrink the market, therefore market forces incentivize competitors to cooperate. It is the same reason that the OPEC exists. It is called game theory, not conspiracy theory.
OPEC is not terribly effective and suffers from constant bickering behind closed doors. The same motives apply - even if there was a "HIV vaccine cartel" there would be a massive incentive to undercut it.
