It isn't only up to skull oxygenation, as you know. G can be a full on haymaker from a very big fist. Acceleration of the skull can lights out the brain in milliseconds. Plenty of O2.
That from the person without experience pulling g.
We are not discussing direct impact to the skull. The Martin Baker back breaker used to give people an 18 g launch until the back injury statistics got out of line. Then they reduced the charge and added rocket motors to spread the impulse. But that 18 g kick did not put your lights out.
Lyman is well known from the AF447 threads, and his strengths appear to be in the legal direction rather than the aerodynamic ones, however he is not hesitant in trying to tell you his version of events, sometimes with fanciful additions.
I wouldn't absolutely discount his ideas, but instead examine them with a great deal of caution.