I read this book (great book by the way), and my impression of the conclusion of Chiang Kai-shek's Northern Expedition was one of a tenuous ceasefire, rather than outright conquest. Don't blame him, he had an army (NRA) of dubious quality and loyalty. This came back to bite him later, when, given this situation, he wasn't able to fight effectively against the Japanese and Communists. Yet when Mao and the PLA came to power, they swept all the warlords before them. Maybe it wasn't as easy as the Communists made it out to be, or did they do something their Nationalist predecessors didn't do?
Its not so much that Mao removed warlords but warlords were more phased out of existence from a variety of factors. One has to remember that the People's Republic of China was only formed in 1949 after a brutal civil war from 1946 to 49 and costal China had to suffer under Japanese occupation. Neither Mao or Chaing Kai Shek had true control over the countryside where Warlords established themselves, instead it was a process of absorption in the case of both the GMD/KMT and the CCP as warlords were offered favourable positions or pardoned for assisting in fighting other warlords. Same goes with the Japanese invasion, warlords helped or were destroyed by Japan and after the war those that aided Japan were also destroyed.
So rather than straight up getting rid of them it was a gradual absorption of warlords and their territories.
Mao didn't get rid of the warlords, if anything it was really Chiang Kai-shek who "got rid" of them. Chiang did not have enough power to defeat all the warlords during the Northern Expedition, so he instead offered them positions in the new KMT government in return for their loyalty to the regime. Many warlords, such as Zhang Xueliang, Yan Xishan, Ma Bufang, Sheng Shicai, ended up fighting for Chiang Kai-shek against the Japanese and the CCP, and almost all of them fled with Chiang Kai-shek to Taiwan in 1949.