This in unfortunately an old fable that stubbornly sticks around. It is not necesary at all to store it in an inert atmosphere - as long as you store it in a small sealed (airtight) bag. The cell will then just use the very little bit of O2 in the bag - and that's it. APD re-seals the cells after testing in a new small bag. That's good enough.
In fact, there is at least one manufacturer here on Rebreather World that says that storing them in a sealed bag doesn't make any difference anyway in terms of overall lifetime. I forgot which one; I thought the guys from Innerspace..? Too lazy to find out.

Look at it this way: if you seal it in a small bag with air, and the cell uses up the remaining O2 in it, it is effectively stored in inert gas, called nitrogen....

ciao,

Tino.