I'm reading the Loeb Iliad so I have the original Greek text alongside English and am confused when reading verse Il. 16-543:

τὸν δ᾽ ὑπὸ Πατρόκλῳ δάμασ᾽ ἔγχεϊ χάλκεος Ἄρης.

... Him [Sarpedon] has brazen Ares laid low beneath the spear of Patroclus. ...

This comes from a speech by Glaucus to Hector as he reproaches him for not defending his allies.

Note the English text probably doesn't correspond exactly to that line, I can't read Ancient Greek and the book doesn't align the translations verse-by-verse.

So, at the beginning of Book 16, Achilles tells Patroclus to don his armour and fight back the Trojans, so that they may fear it is Achilles who attacks them, rather than Patroclus in disguise. Yet at this point, Patroclus' identity has not been revealed (I think), so how can Glaucus know it is Patroclus who killed Sarpedon moments earlier?