Well, I loosen the brake once I pedal kick not earlier. Sometimes I catch myself not letting go...
As for my initial question. I haven't been able to ride lately but went to the gym fairly often. I did a routine of upper body strength, deadlifts and squats. Yesterday I went back to the wall in the above shown video and even though my technique ist still shit, I get up that wall fairly easy. So yeah... strength does help a lot but I guess technique does so even more.