An omniscient agent could still describe a causal structure over the universe- it would simply be deterministic (which is a special case of a probabilistic causal structure). For instance, consider a being that knew all the worldlines of all particles in the universe. It could deduce a causal structure by re-describing these worldlines as a particular solution to a local differential equation. The key difference between causal vs. acausal descriptions is whether or not they are local.
An omniscient agent could still describe a causal structure over the universe- it would simply be deterministic (which is a special case of a probabilistic causal structure). For instance, consider a being that knew all the worldlines of all particles in the universe. It could deduce a causal structure by re-describing these worldlines as a particular solution to a local differential equation. The key difference between causal vs. acausal descriptions is whether or not they are local.