1. East Germany‘s involvement with African revolutionaries began in the 1970s, so I imagine the 25+ years of communist indoctrination trumped the few years of nazi indoctrination that the older soldiers were subjected to in their early childhood. Example: someone most affected by nazi indoctrination would have had to be born in the mid-to-late 1920s to be effectively subjected to their propaganda from '33 until '45. By the first training programs in Africa, this person would be ~45 years old. I think someone ordered to train and directly interact with African soldiers would usually be younger than 45 and therefore not severely influenced by nazi propaganda.

In addition, many activities like the training of Congolese troops in East Germany (1977) happened even later, strengthening the effect of age.

2. The older and higher-ranking officers put in charge of the training programs were given a lot of responsibility. So, I imagine these people had to have had successful military careers and the Soviets - quite strict about their senior officers‘ and administrators‘ ideological beliefs - had protocols in place preventing anyone with a non-communist world view from entering into positions of power.

So, to summarize: Younger officers carrying out the training were mostly too young to be heavily influenced by nazi propaganda, especially during the late operations. The senior officers overseeing the operations were meticulously screened to ensure their 100% communist / 0% nazi ideology.