they're using the css :visited attribute on a link to a HN thread to display the message.
having passively followed asahi for some time (too poor to own a mac to actually try it out) the fact that they have to resort to shit like this is gross, but i 100% put the blame on HN for being an unmoderated trashfire (and explicitly bypassing the referrer based blocking asahi first had instead of, idk, actually moderating their site)