I very much doubt that the tablets will be any good if removed from the original packaging for any significant time.

Rather than debate or argument I would urge an experiment.
These tablets are intended to release chlorine when dissolved in water, if they have deterioted then no, or insufficient chlorine will be released.
Dissolve one tablet in the correct volume of water as per the instructions.
Test the resultant chlorine level using test papers sold for the purpose, I could recomend a UK supplier, but presume that they can be obtained locally.