I'm surprised this has never been discussed.
By 1922, it had seemed that Sun and the KMT had, well, fizzled. The KMT-Warlord alliance that controlled Guangzhou had ruptured, and Sun was cooling his heels in Shanghai.
Enter the Comintern. As part of its plan to promote revolutionary movements, it helped train Chiang (among others), provided equipment, and (some) money. Soviet equipment ad training was key to the KMT's successful wars over the next few years, and without it I don't think the KMT would have succeeded.
What's the alternative, though?
By 1924, Northern China was divided between the Fengtien Clique led by Zhou Zuolin, based in Manchuria, and the Chili Clique, based in Beijing. All things being equal, by 1926 Zhang Zuolin had the upper hand (indeed he had taken Beijing), but he was very much a Japanese proxy.
The end result would be Japanese domination of Northern China, without a shot being fired by their army.