Simple typo in a template, fixed by OverlordQ:
Valid HTML comments in wikitext do not appear in the page source of rendered
pages. It _might_ be nice if HTML Tidy caught this error (omitting an
exclamation point), though.
No. We output a literal <--, which is the correct approach. What made
this non-obvious is that the text-which-should-have-been-a-comment was
inside a block hidden with CSS.
I have no idea what they mean with Taxobox/taxon needing to be warmed
up, though. There's no point on doing that.