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He's not pulling your leg, although I'm not certain that he is entirely correct that "all" printers do it. There is something called printer steganography that is used by many, perhaps most, printer manufacturers. This is a method of hiding information using invisible or barely visible dots. From what I understand, it includes things like the date the document was printed and the serial number of the printer, but it may vary by device. So if you go all Wikileaks, you had best be aware of it or you are going to leave a trail of little yellow printed bread-crumbs that could lead right back to you.
The Electronic Frontier Foundation has at least a partial list of printers that do/do not use the tracking dots.