He didn't ask for his decision to kept secret forever, just until after the inductees had been announced and inducted.It obviously leaked out.
Pretty hard to keep it secret. He's eligible after five years and someone is going to ask why Goodes wasn't straight in.
That would keep the Robbo types, who wouldn't be able to put 3 and 3 together unless they borrowed someone else's hand, from speculating and debating the matter.