I also have a delay set in my profile. I give myself 10 minutes to edit or cool off and delete. But I have never seen a [delayed] comment. That seems to undermine the purpose of the delay.
Maybe the person who saw it as [delayed] is using a third party HN reader app and the HN website itself filters out delayed comments while returning it via the API as the [delayed] string and expecting clients to do the filtering?
I’ve also never seen it in the many users of (mostly many times daily) visiting HN.