Cxiom de, not Cxiom da?
Seems like a strange rule considering quantification is usually "da"...
Da - quantity
De - possession, "of" in English.
So, cxiom da is correct.
And I advise you to read again the Tips and notes.
Good luck in learning Esperanto!
As smart as the dev team is, things can fall through the cracks. That's why we have beta before the course is thrown open wide. Report it as an error and it should be fixed.