It's good to be vigilant against hypocrisy, but there is such a thing as being too focused on possible hypocrisy.
What HN does, i.e., allow comments to contain a couple of carefully-chosen formatting options, is not a realistic option in email whereas because of a history of decades in which email clients could render only plain text, asking email senders to send plain text is a realistic option at least in some situations.
In other words, HN's designers were not restricted to a choice between plain text and full HTML, but in email we basically are (because there is no central authority in charge of email).
What HN does, i.e., allow comments to contain a couple of carefully-chosen formatting options, is not a realistic option in email whereas because of a history of decades in which email clients could render only plain text, asking email senders to send plain text is a realistic option at least in some situations.
In other words, HN's designers were not restricted to a choice between plain text and full HTML, but in email we basically are (because there is no central authority in charge of email).