Given the state of the "historiography" in the area I seriously doubt Neagoe Basarab knew Decebal ever existed. I think he simply took the trouble to write down what was considered traditional wisdom plus his own thoughts (he was a deeply religious man judging by his writings)
The book touches subjects like:
- how to select members of the royal council and assign responsibilities;
- how to make the nobles loyal to the king;
- how to deal with foreign ambassadors;
- how to train your own ambassadors;
- how to fight the war both at tactical level (use of the vanguard, rear guard, scouts, infantry, cavalry, choosing the terrain, etc.) and strategic level (don't try to defeat the enemy in a single battle - wear him off instead, fight a mobile war, always disengage to preserve as much as your forces as possible if the battle looks difficult, etc).
While we don't know how the Dacian kings chose their advisers we know how Dromichete (Dromikaites

) and Decebal fought the Macedonians and the Romans respectively. Same is true for the other kings which preceded Neagoe. So at least the military part of his teachings seem to embody a tradition which was already 2000 years old when he captured it in writing.