Note he uses both, even in the same paragraph - asterisks typically mean "bold," whereas underscores typically mean "underlined."
Except in Slack, which somehow interprets this to be italics.
Markdown started that in general. There's no way to underline something in Markdown; underlining just isn't supported.
Underlining was a substitute for italics before everyone had fancy word processors.
They were always meant for emphasis, which takes on variable definitions over time. Terminals were bad at italics due to character box layouts and so the literary approach of emphasis through italics mutated into emphasis through underlines. Now that we’re all free of monospaced terminal grids, it looks like we’re drifting back to emphasis through italics.
Italics were still used though.
Like /this/.
Back in the days of usenet, and way before markdown, we always used bold, /italic/, and _underline_. You were just supposed to mentally interpret them in your head.
Edit: Ironically, HN is translating my asterisk-bolds into italic.