Can anyone explain what additional claims can be made for the Principle of Compositionality in regards to a language like Basque?

I understand that Basque is a "free word order" language and the position of subject, object and verb can be permuted.

But how can a speaker know the meaning of a sentence if there is no set syntactical structure?

I'm just getting started with semantics so I hope someone can explain it to me simply.