compare the rendering of the test output (which i also wrapped in backticks to tell the markdown rendering to render it as code) on lemmy vs on piefed.

Essentially, it is not possible to reliably identify the URLs in code (in a language that is not known to either PyFedi or its markdown parser), because they be adjacent to URL-valid characters which actually terminate the URL in whatever syntax is being used inside the code block. So, even though it is sometimes harmless to auto-linkify inside a code block, it is also often wrong and therefore should not be (and generally is not) done.