I’m not sure where that NTSC colour space has come from but what I can say is that all the colour gamut transforms everybody is using (Grade, Guest and Sony Megatron) conform to the EBU (PAL/rec. 601 625 line) and SMPTE-C (NTSC/rec. 601 525 line) colour spaces. Just look and compare the numbers in matrices in the shaders vs those in the Khronos Groups pdf - they all match up and aren’t anything to do with this much larger NTSC colour gamut. I dont know
EDIT: Ah-ha! I see where that larger colour gamut is coming from (from the Khronos Groups pdf):
14.5 NTSC 1953 color primaries The following chromaticity coordinates are defined in ITU-R BT.470-6 and SMPTE 170m as a reference to the legacy NTSC standard: Rx = 0.67 Ry = 0.33 Gx = 0.21 Gy = 0.71 Bx = 0.14 By = 0.08 Wx = 0.310 Wy = 0.316 (Illuminant C)
This is much larger - they were getting well ahead of themselves with this colour space and thats then why in the 1970’s everybody moved to the EBU and SMPTE-C colour spaces that the TV’s of the time could actually produce with their phosphors. Obviously 1980’s/1990’s consoles were way past the point of using the 1953 NTSC standard. Everything would look very wrong if we used that input colour gamut.