This relates to the difference between how a real gas and how an ideal gas behave in this scenario.
The difference in this situation will be very small in my opinion, broadly because the gas molecules in the atmosphere still have comparatively high mean free paths and therefore won’t interact with the “escaping” helium molecules.
The difference in this situation will be very small in my opinion, broadly because the gas molecules in the atmosphere still have comparatively high mean free paths and therefore won’t interact with the “escaping” helium molecules.