The issue isn’t whether the HA’s are air tight. The issue is that the chemical in the batteries is activated by air once you take off the tabs. It takes a few minutes for the chemical to be fully activated and reach maximum charge… If you begin to discharge the battery by putting it in the device immediately you aren’t getting the full benefit a fully charged battery and it may run down sooner that it otherwise would.
As an aside, all the talk about how many days this or that battery lasts is fairly meaningless. Different users need various levels of amplification and different HA’s perform different levels of signal processing. My hearing loss may be more severe that yours and require more boost from the device higher drain on the battery. That varies further by how many hours a day use wear the devices.I was told by a rep of Rayovac that modern batteries (312’s) are expected to last 90 hours in most HA’s. Hours is a much better measure of battery longevity than days.