It looks like arborist climbers body-thrusting their way up DRT. A very strenuous way to climb that much height. Guarantee their arms/elbows were aching for days aftwards. You need to get your legs involved doing the work for that much height. It looks like they're climbing a tree that has a lot of foot traffic and is probably an "illegal" climb.
Helmet is personal choice, best to wear one. DRT without cambium protection is a bad idea, it's rough on the tree and it beats up your rope.
I stumbled across this after looking at the video, started a new thread. The climber is using SRT to ascend this very tall redwood:
Climbing world's tallest tree
-moss