You are talking about false negatives, but this thread started with a discussion around the potential for false positives causing writers anxiety
Even if 2/10000 is true that’s nowhere near accurate enough to make aggressive accusations that create anxiety at levels people need to medicate with potentially fatal consequences.
False positives and false negatives are different problems that have different impacts.
Does your single example prove that Pangram doesn't work, or that it doesn't work on short snippets of text? Try to get a detection error on a longer run of text. I'd be curious to see the results, particularly if you can get a false positive.