Interesting idea; for instance a USB-C connector but only with power contacts soldered...
I don't really see an issue, because you cannot hack the device if there are no data lines.

However, power analysis / glitching and such types of attacks may be easier to perform. Though you can also hook up your glitching device to battery terminals, so it's not really that much less secure with such a 'backup energy source'. Would also allow to test for hours at a time without thinking about batteries.

Probably Foundation was going for a 'complete port-less' approach, because people simply may not understand it's fully airgapped if they see a physical USB port on it. Not everyone knows how many pins such a connector has and what it means if you tell them 'we only soldered on GND and VDD'.