Comment by gorjusborg

The permiability of the membrane would still be reduced by stuff on it, but as long as there is any permiability, the inner compartment will reach equilibrium eventually.

The big gain comes from a change in how you interpret the presence of electrons.

The older approach converted oxygen to electrical current, the magnitude of current flow relating to magnitude of oxygen depletion. The assumption built into that approach is that low oxygen depletion levels meant low oxygen levels, but that wasn't the only potential cause, because it ignored variation in the permiability of the membrane.

The newer approach equates current flow to oxygen concentration, as the system doesn't deplete the concentration any longer. The permiability of the membrane in this setup only contributes to a longer initial delay as the inner chamber comes to equilibrium with the surrounding concentration.