That may have been true in the beginning but the blog post says:
"In 2014, I copied articles collected by Libgen to create Sci-Hub's own database. I did not fork Libgen: only the papers PDF files were copied. Then I configured new servers to serve these files to Sci-Hub users.
Today Sci-Hub uses it's own servers to store research papers. However all new articles downloaded by Sci-Hub are mirrored to Library Genesis databases too. Libgen also publishes torrents to download these papers."