Am I correct in my observation that Swift-DocC-Render uses Highlight.js for syntax highlighting?
Yes, DocC-Render uses Highlight.js for syntax highlighting.
Earlier this year, I made a lot of improvements to the Swift support for Highlight.js (Improved Swift support for Highlight.js ). However, that only covered the features I needed at that time, so it's far from complete. If Swift-DocC-Render has made further improvements, can these be contributed back upstream?
Any improvements to the syntax highlighting for Swift (or any other languages) should be contributed directly to the highlight.js project, and DocC-Render can bump the version of highlight.js that it depends on to bring in those changes.