These two movements are done while lying on a bench:
French press - you lower the bar behind your head, so your triceps get stretched nicely
Skullcrusher - you bring the bar down to your nose or forehead area
High rep or low rep, when the triceps engage, all three heads activate to some extent or the other. It is always the strength advocates who will say "This move hits all three heads of the triceps". I've heard that same exact statement said about CGBP, OH extension, triceps dips, Parallel bar dips and god knows how many others.
I'm sorry, I am disinclined to follow your statements based on Pavel's teachings.
In case you decide to dismiss that image, think about it - when using heavy weights, all three heads will have to become fully active; they will have no choice. When using lighter weights, it makes sense that the other two heads (long and medial) won't be fully active (or they may just do nothing at all).
FWIW, I have been doing just heavy CG presses for months now, and my long head is more visible. I barely did any overhead extensions, so I am going to give heavy presses all the credit.
P.S. All I'm saying is that you should try and listen to what I have to say. I'm not parroting Pavel's advice. Use your 3RM while doing the CGBP and see if your triceps grow properly.