We don't know yet if Apple changed anything or not. They could have changed something subtle like the contents of the solder materials, started using a better quality (and more evenly applied) thermal paste, and such a change like that could potentially make a huge difference. Thus far, my machine is running cooler...there are tons of possible explanations for why this is, ranging from Apple cleaned dust out of the fans, to Apple further throttled the GPU, to a better thermal paste, etc.
And I agree with you that we do buy MacBook Pros for extreme reliability based on the name Apple has developed, and the issue with the 2011 15 & 17 inch MBP's with the AMD GPU is extremely disappointing. However, Apple does not currently have any legal liability or responsibility at this point in time. If the computer failed outside of the warranty period, quite often a maker is not responsible...even if a part in it did not meet normally better quality standards given the part itself did not fail within the period the computer was warrantied for.
Whether Apple is doing this to try to end the lawsuits, due to PR, due to their own disappointment with the problem of this design, or because they realize that eating repair costs here is probably a drop in the bucket compared to all the future business Apple will lose if they kept the hush-hush going, I do not know. Maybe more than one of the above, or a completely different reason. But as of now, it is a good will repair.
I also don't think it's completely out of the question that Apple may later change policy and reimburse the people who have had to replace the logic board more than once. Each day we are seeing that the scope of this problem is far more widespread than most people ever would have guessed. And there are still a lot of people that have working 2011 MBPs, many of which will have these same issues sometime in the near future, and the scope of just how widespread the problem is becoming even more well-known.
For those who have had to replace multiple logic boards out of pocket, it can't hurt to write a letter to someone high up in Apple's customer care to first address appreciation for Apple offering this extended repair program, but then also explain their own story and how much money this issue has cost them in total.