I guess Garmin could do something bogus with the air ports and then try to correct in software. But the concept of static pressure for altitude and stagnation pressure for airspeed has been basic to and uniformly applied to aviation flight instruments since the earliest days of flight. It's not new. Somewhere at Garmin I'm sure there is someone who knows this because they've been making aviation GPS units long before they made sports units (I used my first Garmin aviation unit in 1991), and needed to know how GPS indications would correlate to existing certificated flight instruments.

One issue they do have have to contend with is thermal sensitivity of the pressure transducer they use. In basic flight instruments this was done with hardware. Not transistors, capacitors, resistors hardware, but tubes, bellows, gears, levers, bimetallic strips hardware.