I’m no expert either, this is just what I know from experience and through people smarter than me on the internet. I’ve put together some resources below for you. Apologies for the reddit links, it’s just where this content lives.
encoding 10bit reduces the amount of truncation errors during the encoding, which leads to a much smaller filesize despite the two additional bits. To the point where, despite it storing more information, there being less truncation errors offsets the filesize gain to the point where you get similar or even smaller sizes than 8bit encodes.