Could just be a misuse of infamous but could just as well be intended to refer to the fact that Man and Mankind mean the same. You need an article in front to transform "One small step for Man" into "One small step for a man" to refer to Neil himself stepping.
Wikipedia (see elsewhere for link) has good coverage of that. "A" was intended to be said, but when humans say lines like that it is common to miss a word here and there. There is no way to know for sure if he said it and the technology of the time didn't pick it up, or if he misstated his own quote.
