The point is that in a blind test, "a lot %" of the people would never distinguish a freshly opened pack and a pack opened a week before, as much as "a lot %" of the people would never distinguish gold-plated from non-plated ones.
Of course, it's easy to distinguish a "great" coffee [espresso] from a "non-great" one, but then we're talking about a much wider context (that is, many other facts weigh in).
[edit: added note]
Get some whole freshly roasted coffee beans, put one half in the freezer, and grind the other half and let them sit in an open container for two weeks. At the end of the two weeks, grind the beans that were sitting in the freezer. Now brew both sets of beans and do a blind tasting.
You’ll be able to tell the difference, your coffee-drinker friends will be able to tell the difference, fans of instant coffee will be able to tell the difference, and if you have access to a chemistry lab, their mass spectrometers will definitely be able to tell the difference.
It’s completely different from gold-plated audio cables, where you can take very precise measurements and prove categorically that they have no effect.
The problem with pre-ground beans, is that ground beans go stale in a matter of hours. Whole beans from supermarkets are no better since you usually can't tell when they were roasted and they are most likely also stale.