Does a one-turn penalty really have to include both a tack and a gybe?

Yes. Rule 44.2 is explicit: a One-Turn Penalty is one turn including one tack and one gybe. In this case the boat gybes but never tacks, so the penalty was not taken in accordance with the rule — the foul stands. Take the turn promptly, well clear of other boats, and make sure both manoeuvres are actually in it.

▶ Watch on YouTube ↓ Download (via YouTube)
<iframe width="560" height="315" src="https://www.youtube.com/embed/ijOPUF4qrEM" title="Sailing Kiwi" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://sailing.kiwi.nz/v/ijOPUF4qrEM/">Source: Sailing Kiwi</a></p>

Next on this shelf — The Upwind Beat