The strength of the gravity field at the ISS orbit (400km) is still 90% of what it is at sea level. The air thins out at high altitudes because there’s less air above it pushing down, not because of weaker gravity.
To add to this, the force of gravity at the top of Mt Everest is about 99.7% as strong as sea level. So you’re right that it’s not about the strength of gravity itself at that particular point, but about the weight of all the air above that point.
The strength of the gravity field at the ISS orbit (400km) is still 90% of what it is at sea level. The air thins out at high altitudes because there’s less air above it pushing down, not because of weaker gravity.
To add to this, the force of gravity at the top of Mt Everest is about 99.7% as strong as sea level. So you’re right that it’s not about the strength of gravity itself at that particular point, but about the weight of all the air above that point.