what i am working from is advice given from ex-distance record holders and other players that throw 500'+. the only 500'+ thrower i know that uses a grip that rotates their palm to facing somewhat downwards while throwing drivers is tyler horne.
using a version of the grip you have pictured is what caused me to blow out my wrist at the end of '04 as it does not allow for a linear wrist extension (my wrist tried to unhinge and couldn't... it still hasn't healed, will require surgery whenever i get a better job).
the problem with the pictures of brinster you are going from to reference the hand position, is at that point in extension, the wrist will naturally roll under in order to allow for continuation of the upper body. nearly every player that has their right shoulder lower than their left at the peak of their reach will have a similar hand position at that point in the throw (especially emphasized by the movement of the shoulder upwards which is necessary for plane preservation).
the real frames to look at are at or before the rip point.
all of these pictures show the 3 o'clock grip.
i agree that most players have weak grips, and far too many let the disc slip out. 380' can still be accomplished with this though, and it's more of a factor of mistiming for them, not getting enough whip motion from the hips/back, etc.
however, if that grip mod was the miracle that would push lesser players well over 400', it likely would have been discovered long ago. lately i have been working with a slew of ams that had a grip similar to the one pictured and switching them to a one o'clock grip helped them add 75'+ of d with their midranges/putters.
as for tendon bounce, that is a term coined by Dave Dunipace and is more inreference to the phenomenon of plyometric extension (rapid contraction of a pre-stretched muscle aided by tendon elasticity).
the throw itself is still triggered by large muscle groups, but the maximum transfer of force (generated by acceleration) still occurs by smaller muscles. it is the linear opening of the elbow and wrist on their natural hinge points that generates the maximum force (powered by the motion of the torso and legs).
a few years back i used to grip in a 1 o'clock method and sent pictures of it to Dave D. and he wrote back that it was a nose up grip. what i discovered is that it did in fact require a "late release" to get the disc nose down.