Posted before I saw this comment. Yes passive cooling is fine with recent firmware and a good heatsink case. I have stress tested with the following case for hours with all cores pegged at 100% and there is no CPU throttling with passive cooling. They do run hotter, but they do not throttle. Old firware and bad cases will though.
https://www.amazon.com/Geekworm-Raspberry-Computer-Aluminum-...
You wouldn't need that much in their case (sorry) as they have through flow.
The official PoE 'hat' (add-on module) comes with that fan, I believe that's what's shown.
This hat seems to be built for a fanless case. Pretty sure a heatsink protruding through the hat orifice would be enough in their case design.
The fan official PoE hat is way too small so annoyingly noisy, and because it's so small replacements are expensive
There's room for a much larger fan so it seems a case of bad design by Raspberry Pi
That may be difficult with the HATs on every Pi. They look like they limit airflow a lot.