I don't think the author cares about the difference between SIGINT, HUMINT and covert action. He's just looking at sources of information in the abstract. (Like

I don't think the author cares about the difference between SIGINT, HUMINT and covert action. He's just looking at sources of information in the abstract. (Likewise, the author isn't concerned with how the information is used. Only whether or not is it reliable.) If it's cheap and easy for the enemy to produce misinformation that's indistinguishable from the real thing, then it's reasonable to assume that the enemy will do so (and it's a good idea to do so with our own information). Maybe Sorge is a good example of a type of source of information that is costly to fake. He was a trusted agent who had a long career. Maybe it would be far too costly to implant a double agent like that.