I was just watching a video about this, it's pretty cool and honestly impressive that it could be done on N64. They don't even have Half-Life 1 on N64, which makes me wonder if recreating Portal 1 decently in Goldsrc is possible.
The N64 can barely run Quake for PC. And SCummVM.
The GBA doesn't have any 3D graphics hardware, yet it renders that Tomb Raider game with 3D graphics in real time using a software rasterizer, on a CPU with half the clock rate of the CPU in the PS1 and with a fraction of the RAM of a PS1. So no, this is not "just a tiny step down from a PSX".