Speaking and reading out loud are vastly different. Try it yourself: record yourself explaining a concept with zero preparation and then read a text explaining the concept.
Also (I am not a lawyer but) just because the source material is out of copyright doesn't mean the audiobook is out of copyright.
The parent comment was presumably talking about the massive library of unencumbered audio books based on those books, such as those created by LibriVox.
Also (I am not a lawyer but) just because the source material is out of copyright doesn't mean the audiobook is out of copyright.