The poster in question is part of a larger strain of propaganda within the Communist world during the Cold War, namely pointing out America's racial problems. See this Chinese example or this Soviet example for other samples of this particular line in propaganda. The general gist of this line was that behind the glitz of American prosperity was inequities of a permanent racial underclass. The visual language of the GDR poster linked above fits into this mold. The stylized skyscrapers and Statue of Liberty find their counterpoint with the marchers before. It became very important for these regimes to emphasize not only the inequities of the West, but also that there was an growing undercurrent of discontent and protest within the West. As good Marxists, the creators of this propaganda would emphasize the interrelationship of American racism with Western capitalism and all of its various sins. This led to a greater stress on the civil rights movement's anti-war and anti-poverty initiatives within these communist bloc. In the case, the GDR poster takes one of MLK's lines from his "Beyond Vietnam" speech in which he attacks both poverty and militarism along with racism. The poster not only highlights discontent with the American dream, but also enlists MLK into the communist cause. The phrase "revolutionary spirit" was not an uncommon one within Marxist-Leninist discourse and the quote on its own suggests MLK was acting in the spirit of communist parties like the SED.

The poster though is using "Beyond Vietnam" in a disingenuous fashion. What King meant by "revolutionary spirit" was not the same thing the SED was presenting here. The MLK speech was actually quite critical of communism, saying:

It is a sad fact that because of comfort, complacency, a morbid fear of communism, and our proneness to adjust to injustice, the Western nations that initiated so much of the revolutionary spirit of the modern world have now become the arch antirevolutionaries. This has driven many to feel that only Marxism has a revolutionary spirit. Therefore, communism is a judgment against our failure to make democracy real and follow through on the revolutions that we initiated. Our only hope today lies in our ability to recapture the revolutionary spirit and go out into a sometimes hostile world declaring eternal hostility to poverty, racism, and militarism.

So in this speech communism was a symptom of America's ills, not the solution to them.