That was actually the writers and dc who said it around when Flash Forward #6 came out, as no point in the comic did they actually say Manhattans name. They said that apparently when Manhattan killed Owlman and Metron at the end of Darkseid War he left some of his energy on the chair, so when Wally sat in it he gained the knowledge from the chair and the power from Manhattan. It's also indicated by the symbol on Wally's forehead.
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