Here, you omitted the phrase 'known terrorist' from the middle of the sentence you quote on page 22. This totally changes the meaning of the quoted sentence. Also, you chose not to include the following sentence which described how such a presumption could be overcome if the investigative body in question had credible information to the contrary. Finally, you overlooked the fact that 'nominators' in this context refers to other government agencies, relying on existing procedures for designation of someone as a 'known terrorist', and that said procedures form the due process in question. The point of this is to avoid bureaucratic duplication and waste. If the FBI sends the NCTC someone's bio and states them to be a known terrorist (based on facts in said bio), then it's entirely reasonable for the NCTC to take the FBI's information at face value unless it receives information to the contrary.
Such designation is presumptively valid in that situation because the FBI itself is subject to law. If different agencies of the executive branch are required to reject each others' conclusions by default and investigate every assertion de novo the outcome would be ontological paralysis. At such time as someone is detained and charged with terrorist the matter is handed over to the judicial branch, where a court serves the role of finder of fact. Essentially, you're demanding that trial precede investigation.