The note at the top cautions users that the extensions is included after mediawiki 1.26 and so does not need to be downloaded, then says that the other instructions must still be followed. The installation instructions are missing, and are non-obvious. On the readme on github, it says the following under installation:
Add this line to your LocalSettings.php:
wfLoadExtension( 'SyntaxHighlight_GeSHi' );
Using mediawiki 1.35, including the above line worked for me. It should likely be included on this page, though I'm not sure of all of the correct formatting (or if there's a reason it isn't included).