With regards to that picture I think that the low pressure is a very tight vortex with a high spin rate hence the very low pressure. I am no vortex expert though. Annoyingly I can't do CFD and there are no better pictures that I can find. To me the flow lines seem to be circular there, but maybe I am imagining it, the picture is so bad.
In the below image, there is low pressure under the wing, imagine rotating the image along the x axis, such that the vertical axis Z is facing the same way as the Y axis is now. The secondary vortex shown, I believe is the small area of blue on the CFD
"I think that in road cars, vortices don't have a lot to do with how diffusers work!" I would agree with you, I have read very little about vortices in diffusers.
"if it's the large vortex that is responsible for the downforce generated by these diffusers" I don't actually think that it is, I don't quite understand it, so I may be wrong, but I don't think you need large vortices to get downforce.
I actually read a whole doctoral
thesis a few months ago about diffusers on Saab cars and there was no mention at all of vortices within or at the edge of the diffuser.
I think vortices are only really relevant with low ground clearances and high pumping ratios. A very low car at 15cm ground clearance would need a diffuser that is 75cm high to have a 5:1 pumping/area ratio. And realistically there aren't any road cars that would have a diffuser that big.