Yes, it's pretty much a mentality training IMO. Like he said, "getting a new personality". Instead of getting his power from his old, not allowed ways, his "post shoulder injury" self embraced the clean path to improvement... And this training, which some analysts faced critically, like if it were all a gimmick to distract people from his continuously "wrong, illegal improvement path"... A "performance boost washing" of sorts. But it wasn't, it was a legit change in his approach, which is directly related to getting himself used to the hellish solitude, to the darkness. To have a moment to be feel comfortable and embrace the darkness.
Jiri would say that he would have nothing but his own thoughts right there, when meditating for days in a dark room with only water. He'd be faced with, likely, a storm of thoughts from a fast paced mind... But through commitment, sheer will and discipline, he'd conquer that, what I imagine was a torturously uncomfortable feeling, thus growing mentally... He'd feel more comfortable in whatever environment he was in, with the "pure performance/ pure, natural focus". "Not any other bull shit, just a pure performance, a perfect storm". Not an artificially boosted storm, but a pure storm that came with personality sacrifice, life sacrifices... All a process toughen up his mind.
After all, ppl don't know, but many times those energy monster drinks boost are way more mental than physical. Which doesn't mean they don't help, they do help a lot... But because the mind gets used to operate at its best only under that "good motivation feeling of pushing harder"... That big focus and mind clarity it gives you, so you can train as hard as ever with no distractions, fully immersed and enjoying the process. Changing a many many years habit, for some pp, works only if done in a cold turkey way — a way that will hurt, that will lead to anguish, I imagine. But when that anguish, that pain, that self questioning — even that "who am I?" — get tamed and totally controlled... That's when his training benefits start to improve over and over, needing only "natural food" to push in a hard and effective way... with your own mind producing the dopamine for it instead of a Monster Adderall not only recycling it, but also shooting additional dopamine in your mind almost to the point of overdose...
This short he posted of him screaming in the beautiful, nature, but also lonely waterfall landscape... Might symbolize what everyone took at, at the time — Jiri channeling his strength... Or instead, actually, his own despair and anguish, releasing that pain inside in a huge roar, of how hard it is to reshape the mind and embrace the heavy sacrifice.