I’m scratching my head as to how they pulled this off without resorting to distillation rather than just fermentation. Even the strongest brewing yeasts I’ve known of, could only reach around 12 or 13 percent alcohol. For that matter, I’m not aware of any wine or mead yeasts that can reach 82 proof. I’m wondering if this brew was “fortified” with additional alcohol after fermentation?
However they did it, 41% alcohol isn’t going to taste remotely like beer. That’s hard liquor, and likely not very good tasting liquor at that.
It's the 'Eisbock' (ice bock) process, where the brew is frozen and ice removed, leaving the rest with a higher alcohol concentration. Guess in a way it could be considered a form of distillation.