I was wondering the same question, and IIRC it's due to the DNA diversity we have today. This is roughly calculated with the successive generations of humans and accounting possible mutations.
If this is the case, how can we know whether that number represents a drop in population or not? Can we measure population size 100,000 years ago? 50,000? Then compare them? What about that number represented as % of total population? 1,000-10,000 sounds low, but our population probably wasn't very high that long ago either.
Hypothetically if you took a random sample of present humans you would have a low # of say skin tones in wide range, whereas a small isolated and stable population should have a low # of low variety. So you can tell by that kind of difference whether it was a sudden drop or more of a sustained thing. Presumably in a more sophisticated statistical way across a wide variety of genes of course.
Apparently there is another bottleneck ~900K years ago that has decent fossil record support but the Toba one is more disputed.
