Well I don't agree. What you are explaining is that the two engines running simultaneously are a "physics + logic" engine (the original one) and the graphics engine (the new one).
Of course a remake can be made without rewriting everything from scratch. This is how everyone does remakes: they improve textures, models, resolution, etc and this doesn't affect gameplay at all because the logic part stays untouched.
What I have
read is they are running two graphics engines simultaneously to provide a seamless switch between the new look and the old one.
If the old engine running simultaneously was really only physics + logic, it would mostly affect CPU load, not GPU load and therefore it wouldn't justify a drastic drop in resolution like that. If they drop the resolution from 1080p to something like ~900p its only because they want to save GPU time for something else... Like running another rendering engine simultaneously.
So it's really a gimmick, a weird design choice but fan seems to like it so I guess it's OK.
I would have personally prefered to pick which engine I want to run in some kind of menu before starting the game in favor of a greater resolution.