I don't think that's likely considering that they are mostly laying down in the car with the feet only slightly below their head, so relatively flat overall. In braking, blood would be forced to their legs/feet, so you'd see gray/black out.
You might have gotten it right. I believe the major cause of redout is blood rushing into the eyes/veins in the eyes bursting. The head is still more or less level, even if the body isn't. So it's still very much "eyeballs out" G.
Breaking G is also more or less required for the drivers to break properly.
All breaking force has to be generated by the driver as per regulations, and the pedal requires over 100kg for maximum breaking, all while modulating the force precisely to extract the maximum without locking up[1].
So they rely on the breaking G forces making their legs heavy.
Cornering G is lateral.
That doesn't pull blood from the brain. Not to say it's exactly FUN, but it won't cause you to black out.