I placed an internal wikilink in the summary, to provide context for my moderation action. That wikilink should have worked.
I've asked, and been told that the usual edit-summary uses "Linker::formatComment" as the limited parser. That doesn't seem to be documented at a technical level anywhere on mediawiki.org.
The user guide to "what works in edit summaries" is at https://meta.wikimedia.org/wiki/Help:Edit_summary#Properties
From merged task:
Per T95174, moderation reason fields do not support wikitext. This breaks user expectations and make the explanations less useful than they should be: