Ignoring the order could result in personal liability of people involved in the project (as this is now a willful violation). Those people can then strike off Germany (and likely the EU and other countries) from the list of places to ever visit.
They can seize all assets within their reach (e.g. donations from Germans) but this would
be a larger problem if the site would be a for-profit company with European customers. So the risk for the site to have the ruling actually enforced is lower than usual.
The judgement clearly states that the books in question should be blocked from being accessed from Germany. The court is aware that such blocks can be circumvented but that's still fine. They never claimed jurisdiction over what happens to users outside of Germany and explicitly haven't ordered the site to take down those books completely.