If somebody had asked me what design movement this was inspired by, my first answer would have been that it's an interpretation of Frutiger Aero [1] through the lens of Windows 3.1.

But if you change the theme to grayscale I can kind of see the connection to brutalism. The visible div borders around every comment are bare structural elements, and the huge skeuomorphic buttons are certainly closer to brutalism than HN's UI. HN is about the UI being minimalistic and muted, out of the way to the point of being nearly invisible. Brutalism on the other hand is about a kind of in-your-face minimalism, which this version does capture in a way.

1 https://aesthetics.fandom.com/wiki/Frutiger_Aero