If you dig into the schematic, all three modes are Mark IIC+ circuits.
In Mark IIC+ mode it has the small coupling cap of non-GEQ models, and PULL DEEP is disengaged.
In Mark IV mode, it has the larger coupling cap of GEQ models and PULL DEEP is engaged. And in fact, the Mark IV didn't even have a PULL DEEP circuit that matches, so it's not even an exact Mark IV recreation, but it IS a IIC+ recreation.
EXTREME just changes the negative feedback in a way that didn't exist before.
So honestly, I think people just compare to the one on the label because they chose to label it like that and haven't dug in to realize they're both IIC+ modes. Honestly they should have called it Tight, Deep, Extreme.
But they wanted to market the IIC+ legacy. But if they called it IIC+ tight, IIC+ deep, extreme, then someone would go "but I like my mark IV, what if I can't get my sound". But you're right, I think more people should be compariing the Mark IV mode because if you're into heavier tones it was probably a GEQ model and PULL DEEP engaged. And the manual could have used a "for comparing to IIC+ with GEQ use Mark IV mode and do this" section to go beside the one for the IIC+ mode.