Comment by thaumasiotes

To be more specific, that usage note strongly suggests that the problem is in the font used by HN. The font is what complies or doesn't comply with the Unicode standard. We can also say that HN has a problem, but HN's problem is "they're using a noncompliant font for monospaced text".

(On further investigation, I got the characters backwards, and HN's ordinary display is correct while the monospaced display isn't.)