Yanlış değil, Youtube yorumlarında da senin gibi anlamayanlar var, biri açıklamış al belki yardımcı olur.
Your problem is with the data given by the doctor: "among those who are positive on the test, just 1% are falsely diagnosed." This is not the information given by the doctor, though I also thought this at first, and I've seen this mistake in other comments. In fact, I think this is the misstep that Derek is trying to highlight.
The doctor actually gives two Separate pieces of data: 1: "the test will Correctly identify 99% of the people that Have the disease," and 2: "it will only Incorrectly identify 1% of those who Don't have the disease."
This sounds like he's addressing two halves of a population, first those with accurate results, and then those with inaccurate; and, you would expect the two halves to add up to 100% (99%+1%=100% Yay!).
Except these statistics are actually about two different populations, those With the disease and those Without. First, of those With the disease, 99% will get a True Positive. Second, of those Without the disease, 1% will get a False Positive, meaning 99% of those Without will get a True Negative. Each statistic gives 99% accuracy for its population, together providing the overall 99% accuracy of the test, which is what Derek says most people will use as the accuracy of their Positive Result. Except the accuracy of the Test is not the same as the accuracy of the Positive Results, because the rate of Sick people getting Positive results(99%) is not the same as the rate that Positive results go to Sick people(Because some go to Healthy people as False Positives)...
Where are the positives going: 99% of the Sick people get True Positives and 1% of the Healthy people get False Positives. But, there are 999 times more Healthy people than Sick people. So, from 1000 people, you would expect (1person)(0.99accuracy)=0.99~1 True Positive and (999people)(0.01inaccuracy)=9.99~10 False Positives. So, while 99% of Sick people get Positive results, only 1/(1+10)~9% of Positive results go to Sick people.