I'm doubtful about the accuracy of any battery test that can be done in minutes over the counter, even without concern for the guy screwing things up. So in my belief, results from there will often NOT be definitive and conclusive. Good enough for many, and often it's the best you can reasonably do, but ...

What I do whenever I can is to get my fleet set up so I have cars with interchangeable batteries. So whenever I get a situation where I'm getting weird battery stuff, after doing all the easy testing, charging, etc. I can do, if doubt remains, I just can swap batteries between cars and see if the problem goes away.

Not much harder than taking a battery into the store for questionable testing.

It does take a little effort up front, when buying new batteries for each car. Sometimes I run with a slightly smaller or slightly bigger battery (that obviously still fits and works), to enable this more definitive testing.

For example, the battery in my 2001 X5 is exactly interchangeable with the one in my 2004 Cayenne. I have not done it yet, but if I ever think one of the batteries has a problem, I can swap them pretty easily to see if the problem stays with the car or with the battery.

And for diagnosing simpler battery problems, I've found the most telling failure is that when charging up a battery on my 25A smart charger, it goes from dead to full very quickly (minutes vs. hours).

BTW, this method helps for general electrical debugging as well as battery management. Many electrical problems present intermittently, and can be tough to isolate. Very often, a correct and wise decision may be made that the problem could be due to a weak/bad/intermittently failing battery. So the step is taken to buy a new battery and see what happens. Which is great, unless the new battery did not fix the problem (although it often improves things), and you've done nothing but spend $$ and remove doubt about the new battery ... unless the new battery needs to be doubted too. By swapping in a battery from a second car with no problems, it is already known to be good.

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2001 X5 3.0i, 203k miles, AT, owned since 2014