I tried to use get_discussion_message. .id was determined correctly. but .text and .author belonged to the post itself.

ch = -...

app = Client(
    "my_account",
    api_id=...,
    api_hash="..."
)

async def main():
    await app.start()

    async for message in app.get_chat_history(ch):
        try:
            msg = await app.get_discussion_message(ch, message.id)
            print(msg)
        except:
            None
    await app.stop()


app.run(main())