call it his Mentor's practices rubbingoff on him, but Nightwing wont trust the information/take teh information with a grain of salt. If there was that level of information about Slade, its either a ploy to throw them off, or an outright trap, which has happened before, part of the source of his outburst. By his logic if there was that level of information floating around, Batman likely would have even gotten it before anyone else.