On big other is able to compile with small or not work on a range of less popular different platform, but again, I am really not sure what it has to do with what was discussed here.Available for you to install is NOT the same as something being open source. One of the key aspects of open source software is that it can be altered freely. You cannot alter DLSS to run on AMD hardware without nVidia's permission.
It is not like there is a list of the most popular fork of FSR 2 outthere or that alteration of the code was relevant to the subject.
Take Xess I am not sure if they made it openSource over time (seem like to still be only the header and not the code on github), but the API was an open Standard, which would be more important here for the competition to be able to do upscaling when a player set DLSS on in a game (or simply called advanced upscaling).
That the internal source code optimized for your hardware of the upscaling task is opensource or not, would the studio making a game particularly care ? What does it have to do ?