We have a list of explicitly blocked IP ranges. However, a configuration oversight on our part only blocked these ranges on the "normal" routes. The "anubis-protected" routes didn't consider the challenge. It was not a problem while Anubis also protected from the crawlers on the other routes.
However, now that they managed to break through Anubis, there was nothing stopping these armies.
It took us a while to identify and fix the config issue, but we're safe again (for now).