They use LLMs to analyse social media and turn unstructured text ("I always get stuck waiting for a hook turn") into structured data (location=Melbourne, Australia).
They test this on a set of Reddit profiles that have been hand labelled.
However, the fact that this hand labeling is possible indicates in itself that the issue isn't a pure privacy violation. It's more that automating this in bulk is now possible which was difficult before.
Is this ethical? It isn't obvious to me that it's not - I think most people writing on a public site realize their writing is public, and thew fact we see common warnings about not posting personal information shows that is true.
It's not clear it violates privacy expectations either - even if some people don't like it.