Could you please provide a link to that?
Searching the SE–0169 review thread for “private extension” yields zero results.
My recollection is that it was not brought up until after the review. And in particular, that it was you yourself who first mentioned the issue in the SE–0169 acceptance thread.
The first response to that, 2 posts later, by the review manager themself, was a resounding Yes:
It does imply a change to "private extension" semantics: the members of such an extension would be "private", so they'd be visible to other extensions (and possibly the definition of) the extended type that occur in the same file.
The next post, by another member of the core team, claimed that doing so would also require declaring top-level functions and typealiases with fileprivate, which seems like a complete nonsequitur to me.
private function at the top level would obviously still be synonymous with fileprivate function. It is only private extension that would gain its obvious meaning of “the members of this extension are implicitly private”.