At some point your thumbs wouldn't activate the pads so you had to use your thumbnails and then it was just a matter of time before the tester strip quit working.
It's interesting that he speaks of them as something very old, but such batteries were still widely available in Europe not too long ago. I have a pack of Duracell PowerCheck AAA batteries made in Belgium and labeled good until 03/2029, which suggests a manufacturing date of 2019.
Imagine AA batteries with little LCD screens.