What I'm getting from the article is that to work around the lack of clamping in the RDP, it is set up for additive rendering with low intensity onto a 32-bit framebuffer to make use of its higher dynamic range. But that's way too dark for display, so the RSP is used to scale and clamp this down to a 16-bit framebuffer that is then displayed.
Using the RSP makes this possible at a reasonable framerate, but it requires writing custom microcode for it and Nintendo didn't provide the necessary tools or documentation back in the day.
My read is that this makes pseudo-HDR rendering techniques possible on the N64.
