> what's basically the sum total of human scientific knowledge
That’s an exaggeration. First, a lot of scientific reference books are not a Sci-Hub. Second, decades of published materials have not been digitized and are not available there. Third, very recent work (2020-2021) are often missing as well. And finally, there is a biais towards English and articles in other languages (for instance Korean, Chinese) are often not present despite being indexed or available on other networks.
Sci-hub is awesome and I use it almost daily but it’s not perfect yet and it’s not near "total sum of scientific knowledge" yet either.