Very few mag mounts have an adequate "RF" ground, at least on 11
meters or lower. There isn't enough capacitive coupling to the
vehicle body. In some cases the length of the coax can matter, not
from a matching point of view, but as a counterpoise for the antenna.
I have seen mag mount installations where you could see the SWR change
just by moving the coax around.

Please explain how a poor "RF ground" can be corrected by increasing coax
length??

Don

He never said that a poor RF ground could be corrected by coax length.

He said in effect that the side effects of a poor RF ground could be
reduced by a specific length of coax.