I'm just guessing and am in no way a graphical expert, but maybe that's why this port is even happening? Game is very linear, not a lot of enemies on screen, and I imagine the parts where the graphics really excel over other games (the parts I bolded) are going to be cut down significantly, if not removed. If I recall correctly, there's been a few Switch games which had to change the lighting system to compensate for the lack of power, for example.
I also have to wonder if the version shown at the direct is even the Switch version, or just what a Switch version could/will theoretically look like.
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.