That's simply not the way it works at all. CDPR has absolutely no input on what another company does with their hardware or software. It may be incorporated in the future, but a studio has no
responsibility to incorporate this or that technique officially (things such as ray tracing, Nvidia's SLI or AMD's Crossfire, whichever frame generation option, etc.) This is why there are still games that don't use ray tracing, that won't take advantage of multiple GPUs, or that don't recognize a certain method of supersampling or frame gen. Not all engines are going to react well to one manufacturer's technique or another's, and it's not any studio's responsibility to get them working. These are options provided by the GPU companies to override a game's engine and offer better performance, cleaner images, etc. It's their responsibility to get those features working and support the games, not the other way around.
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What I can confirm is that every stitch of information online now about the Intel XeSS SDK 2.1 confirms that
XeSS-FG will not work with Nvidia DLSS.
You must use XeSS with XeSS-FG.
Even if the option is enabled with DLSS, there is a general consensus that it's not actually affecting the FPS in-game. Furthermore, to use the XeSS options, you must have a GPU that uses the DP4a instruction set and Shader Model 6.4 or higher. That means A GTX 1000 series for Nvidia or Radeon RX 5000 series, though Intel itself recommends an RTX 3000 series or Radeon RX 6000 series or higher. For Nvidia, the card must also support Pascal tech, and AMD cards must support RDNA. (I believe there are budget models of the lower-end cards on that list that do not support Pascal/RDNA, but those will be much older cards.)
What is not listed anywhere on any information from Intel is a statement that XeSS-FG will work with Nvidia's DLSS.
XeSS uses its own pipelines to achieve the effect. SR, FG, and LL all require XeSS, not some other company's approach. The SDK 2.1 simply allowed
XeSS technology to work more readily with AMD and Nvidia
GPUs -- not their technologies and approaches. It
replaces their technologies and approaches.
Hence, if players have a compatible Nvidia or AMD GPU, they can choose to use XeSS
instead of their GPU's native supersampling and frame gen. They may not
combine the technologies together. They must select one or the other. (And even then, there will be bugs. It's new tech. There are always bugs.)