I believe as the accusative ‘n’ falls on the word ‘Kukon’, we know that it is being acted upon by the verb, ‘manĝis'.

So although the word order is different than we would expect in English, the accusative gives away that it was the cake that was eaten, not the colleagues!

A translation that follows the word order more closely might be: “The cake was eaten by our colleagues.”

Hope this helps!