> that partial pressure against the breathable air is of the same order of magnitude as breathable air against a vacuum. The engineering challenges are similar.

Those are very different engineering challenges. Separating similar pressure gas mixtures is question of porosity, but not force. Separating different pressures of the same gas mixture is not just a question of porosity, but you also need manage the forces created by the overall pressure differential.

Edit: If you take a helium balloon into space (or high enough in our atmosphere) it explodes rather than just slowly leaking and equalizing it's gas mixture with the atmosphere.