Same for me, copying out of a variety of editors/IDEs and it doesn't retain the font/syntax highlighting at all.
To be fair I didn't really expect it to work with automagically copying the syntax highlighting from my IDE, but it made it sound like it could.
It's definitely a useful project, and surprising that nothing else like it exists, but may be worth making it clearer that it's designed for rich text, and may or may not work with formatting when pasting for various programs.
Atom: https://atom.io/packages/copy-with-syntax
VS Code: https://marketplace.visualstudio.com/items?itemName=atian25....
Sublime Text: https://github.com/n1k0/SublimeHighlight