Sadly, security issues are still being found in sudo, so wasn't broke isn't entirely true. Though, whether or not Rust prevents a given security issue is strongly dependent on the kind of issue. Security issues arising from logical errors usually don't get caught, there is only a guarantee for memory management issues.

missing some configuration features of base sudo

One of the things sudo-rs does is implement only a subset of features to decrease the attack surface. A recent security issue did not affect sudo-rs because they simply did not implement the feature that had the (logic) bug. As with many things this is a trade-off.