They say pretty directly in the post that they didn't want to deal with the hassles around dongles and uncommon ports for using this as a Linux PC in their pocket.
Imagine you check into a hotel, and want to use the in-room TV as your display. There is probably a set top box there with an HDMI port going to the TV. You would be able to unplug that and plug it into your Flipper One because it has a full-sized HDMI port.
Go to any store, and look at what cable they use to connect their POS computer to the display. It's probably HDMI.
For better or worse, HDMI is extremely ubiquitous.