I only have a fairly basic understanding of encodings, but is this one by any chance in one of the smaller “standard” character sets and not in the huge Unicode “emoji” set?

It is not an emoji but a unicode character and HN doesn't render it, your OS does.

Not up to HN, it’s up to your font and font rendering.

this is the first emoji I see on HN at all, they’re filtered out in comments.

first time also! depending on iOS for me.

that's specific to your os it renders in os emoji colors iirc.