Still the original Doom features are all there, except multiplayer. They also restored some missing graphics features of the GBA port, like z-depth lighting.
Yes 4MB vs 108kB is more impressive than 256k vs 108k, but cutting in half the memory requirements is still notewhorty.
So the OP is not entirely wrong: they ported a game played on a 16.8Mhz system with 256kb of RAM to a 80Mhz with 108Kb of RAM.
The writeup explicitly says «we could trade off some computing power of the 80Mhz Cortex M33 to save memory»