The page says: "This extension comes with MediaWiki 1.21 and above".
Does this mean that I can use the <syntaxhighlight> tag in something that another extension outputs and be sure that it will work?
Topic on Extension talk:SyntaxHighlight/Flow
Unfortunately, no. It just means that this extension is included in the Bundled extensions and skins tarball bundle. It still needs to be enabled in a wiki deployment's LocalSettings to be usable.