It was more that there was a gentleman's agreement with the main White House correspondents not to report it. People knew that he had weak legs and would sometimes use the chair; the big secret was that he couldn't really walk at all.
His big public appearances were the ones that were elaborately staged to make it look like he could walk (one of the more famous ones being his keynote at the 1924 Dem convention, where he rehearsed walking onstage with the aid of his son James, made it to the podium, gave a rousing speech, and then, in the midst of all the applause, somebody in his entourage realized they hadn't given any thought to how he was going to leave the stage).