It appears to contain basically all private and public games/tools/trailers on steam between 2003 and 2013.

10 years of all games' content is only 13TB? They are 'ancient' games, but I still find the number too small.

Maybe from a decommissioned content server? Some of this stuff was publicly available up until 2013 (people were frantically downloading old builds of stuff in the lead-up to the Steam2 shutdown in 2013).