They say: what happens in a chaotic system can usually not be calculated.

That is probably correct - because in the initial state often not everything was definitely fed in and it is difficult to foresee interactions with substances outside the system.

But: Chaotic systems consist of substances.

All substances run according to laws *.

The definition for laws is: Identical substances under identical circumstances always produce an identical result. (Or to put it less precisely: the same substances under the same conditions always result in the same.)

Conclusion: Because you cannot calculate something due to infinite interactions, that does not mean that everything does not follow the law here.

So, you don't have to (be able to) calculate everything. It is only important that you know that everything consists of substances that operate according to laws - in order to realize that chaos is also in order.