I guess the writer doesn't see the irony in his own use of the past tense.
Yeh. I think he’s referring to the “invincible” Sacred Band of Thebes. Which was utterly wiped out by Philip of Macedon in 338 BC.
The Sacred Band of Thebes was not invincible against Phillip of Macedon. Every one of them was (supposedly) killed in their final battle at Chaeronea.
To be fair, they supposedly all died without retreating a step, so that they died but were not defeated.