I don't understand what you mean by "honour the mileage chart". When you book a flight via the Skymiles program, Delta takes your miles and pays money to the airline partner to buy that flight for you. The airline partner could not care less what the mileage chart says. They get money. The mileage chart just specifies the amount of miles in exchange for which Skymiles/Delta is willing to buy you a flight from their partner.

And what do you mean by "secret"? The fact that you can spend miles on flights with partners (particularly Skyteam airlines) is openly mentioned on Delta's website.