It isn't quite that extreme, but it would be a massive undertaking. It would require a lot more work than what Panic Button and the like usually do. It would need actual asset rework going beyond texture resolution. If you look at Dragon Quest XI S, you can see kind of an example of what would be needed. The Switch version features a lot of newly created versions of various assets. Of course, that doesn't address the CPU issues, which is the real challenge for an open world in the style of Witcher 3. Which is where the game might be able to juuuuust about almost get by on the GPD Win 2 despite the glaring GPU deficiencies vs the Switch.
Yeah I guess what I should've said was not so much that it's flat-out impossible, more that it would take such an extensive reworking of the game's technical makeup that I can't see a third party investing that much resources into a Switch port. You couldn't just turn down the settings like with Doom or Wolfenstein II or Hellblade.