CYPRES AADs work based on calculating rate of decent, not from some triggered barometric pressure. With modern micro-controllers it's practically trivial to tell the difference between a barometric altimeter sensor history of pressure changes in a cabin and pressure changes due to free falling through the open atmosphere. From there it's just a matter of triggering based on rate of decent, altitude ranges, etc.
Note how they call out that you can keep it activated on the ground, in vehicles, in pressurized cabins, etc. without fear of accidental automatic activation.
Rate of descent is calculated from barometric pressure in a CYPRES.
When a CYPRES is turned on, it calibrates itself and uses the current barometric pressure to determine the reference ground. When you enter an unpressurized aircraft and begi your ascent, the CYPRES will arm itself above a certain altitude (1000 ft normally). When you exit the aircraft, the CYPRES knows your altitude because of the difference in barometric pressure between the reference ground and the urrent pressure. It will also calculate your rate of descent from change in barometric pressure over time. It will activate if you're too low and too fast.
Notice how the environment is fairly static. You're jumping right above your reference. You're in an unpressurized vessel so the CYPRES can tell when to arm itself. You're starting a descent by exiting a level-flying aircraft. Any departure from normal operating limits and the CYPRES will not arm itself or activate at the proper speed and altitude.
You want a system like fighter pilots have and not something skydivers use. The environments are different, the operational requirements are different.
Some "selling points" of the CYPRES-2: http://www.cypres-2.com/
Note how they call out that you can keep it activated on the ground, in vehicles, in pressurized cabins, etc. without fear of accidental automatic activation.