That page isn't a complete listing, but you're right, only ~100 of the books seem to be properly open access, unfortunately. I guess Springer can decide to revoke access to the rest at any point in the future.

Edit: looks like they've revoked access already in fact https://news.ycombinator.com/item?id=10811854

I should have downloaded a few more, in retrospect it's clear it was too good to be true... Especially when no announcement had been made.