It is not a rare type of strength that allows him to hold this joint angle, but his lower body mass and the arch in his back.
The more open shoulder angle is a lot easier and he uses it because this is obviously his point of balance, and is effected by the placement of his center of gravity. Going on his fingers even further proves my claim, because he decreases the use of his wrist to create any force to help change the shoulder angle to a different one than the one dictated by your body structure. Even with a flat wrist, I do not believe it can be changed very much, great forces are in action here.
Alos, there are other issues that should be considered here, and also while starting to transition to a maltese training: (or moving up in the planche variations)
there is some change in the musculature in use when changing the shoulder angle to a closed one. I believe part of the problem in transitioning from all the different planche variations is this reason. You are slowly engaging new, small muscles into the equation and making new demands on the system. (inter muscular coordination, intra muscular coordination, strength, etc...)


