It becomes tricky though when a lot of your game's core rendering tech is entwined closely with the design of the graphics; to the point where it's not as easy as just switching off effects as in a lot of cases you'd have to rebuilt all or most of your assets to prevent the art design being fundamentally compromised.
I think what we saw in the Direct was work-in-progress Switch footage; it definitely looks softer than the existing console versions and in the scene with fire you can see its using low resolution alpha with a reduced fresh rate, which is a common cutback in Switch ports like Doom due to the system's lower bandwidth.
That's true but I think it's very likely that either the engine is more adjustable than we think, or they are making stuff to compensate for the limitations. The port has been in development since early 2018. Which means that when the game launches, it could have been in development anywhere from a year, to a year and a half. That's a long time for a port, even if the team is small. So I do think it's somewhat likely that, even if your scenario was true and it would be hard to just adjust parts of the games assets, it's possible they did in fact make new lighting/effects for the game. And by "new" I just mean "worse", lol.
That's funny, I was just going to mention the fire sequence