Post-hardcore is a punk rock music genre that derives from hardcore punk but emphasises a greater degree of creative expression initially inspired by post-punk and noise rock. Like post-punk. – Wikipedia

Breaking Down The Word: Post-Hardcore
Post – From what ive picked up, is when we use the word Post while dealing with music, it means less aggressive (at least in performance) and more possibly more experimental.
Hardcore – Hardcore is actually hardcore punk just without the “punk” at the end. So hardcore punk is a punk rock genre. Its generally faster, harder, and more abrasive than regular punk rock.

Punk (which post hardcore is related to) often has anti-established lyrics. You find anti-established lyrics all over the place but punk gernes lyrics are mainly made up of them.

Hopefully this answered your question and I do agree that the post-hardcore label is somewhat widely used but I do strongly believe that it does have meaning and i think you can agree on that.