Companies can't "automatically" void warranties due to using other parts with their parts. Sorry, but that TOO is included in the Magnussun-Moss warranty act. If they could do that, then every power supply manufacturer could void your warranty because you plugged their power supply into another manufacturers graphics card, or because you purchased custom sleeved power supply cables.
Now, that being said, if they can PROVE that THEIR product failed BECAUSE of a third party part, then they can refuse to cover it under warranty, but that applies to everything warrantable that you can buy. Everything. If you buy a refrigerator and plug it into a wall socket and the refrigerator fails and they can PROVE that it failed because there was a fault with the socket itself, they aren't going to replace it. But trying to prove that costs companies more than simply replacing the product, so they don't bother, because more and more they are learning that they WILL have to prove it and that it is going to be costly and time consuming.
And, again, it doesn't MATTER what Nvidia says, because they are beholden to the law just like everybody else. It would be like car companies trying to tell you that you can't put any tires on your vehicle except the ones that THEY sell, or your warranty is void. So, you can all believe what you want, but anybody who thinks Nvidia has the power to tell people they can only use their PROVEN faulty adapter, clearly does not understand the laws in this country. For other countries, it might be a different story, although, there are a lot of countries with even MORE stringent consumer protection laws, like Australia among others.
And you can bet that Seasonic, Corsair, Silverstone and others will likely be publicly challenging Nvidia on this issue soon, especially if Nvidia doesn't immediately begin a replacement program for the proven faulty parts with shoddy build quality. And they will win, because if Nvidia loses the relationships they have shared with all of the major power supply manufacturers, they are going to have a very bad day, metaphorically speaking.
Can you imagine how, and this is not a terribly different situation, things might have turned out for Ford on their 3 valve engines if they had started telling people "You can't use spark plugs that have been redesigned to not break off in the cylinder head, you have to use the ones we've included even though they are costing customers thousands of dollars in repairs" (Before the tool to remove the broken parts was created, of course). Yes, you can imagine that there would have been enough lawsuits to keep Ford broke, and busy, for decades. The words "class action" come to mind.
But by all means, use the parts that we already KNOW have been shown to (In some cases, not all to be sure) have problems, rather than the ones which are much better built and will likely never cause you any kind of problem at all and will therefore fall under the "moot point" consideration when it comes to warranty concerns since they are not built like crap, like the Nvidia adapters.