HAVEMANN countered this by turning the condemning word around and presented MARX as a “revisionist par excellence”
And what does it mean to turn the condemning word around?
Context is everything. To understand this passage you not only need a score card to keep track of the players, you also need at least a rudimentary knowledge of the game and its history. The game here is Marxism, which has a long history of convoluted expression in many languages, including Russian, French, English, and the original German.
Your source is an article Being a Marxist by Wolfgang Fritz Haug, a professor emeritus of philosophy at the Freie Universität Berlin. The article is from the journal of the Berliner Institut für kritische Theorie, and it discusses the history of the arguments over what it really means to be a Marxist. To answer your question, we must back up a paragraph in the article to find the following:
[Polish Marxist Adam] SCHAFF found the meaning of the question of what b[eing]M[arxist] means defined by the complementary question "what it means to be a revisionist."
That is, Schaff explored the question of what it really means to be a Marxist by investigating revisionism, i.e., the alternate interpretations of Marxist ideas, interpretations at odds with the orthodoxy of those in power.
[T]his category [i.e., revisionism] in the epoch of Stalinist-shaped M[arxism-]L[eninism] in power no longer designated the abandonment of the emancipatory basic impulses and with them the goal criteria but could mean everything that contradicted a leadership or its ideological guardians.
Here you have to know that the original Marxism espoused the "liberation" of the proletariat (the working class) from the "slavery" of capitalism ("emancipatory basic impulses") by establishing state control over all economic activity ("goal criteria"). So revisionism in that context would mean questioning this basic tenet and strategy. But when Stalin and his followers came to power in the USSR and eastern Europe, they redefined revisionism to be any challenge to their authority.
We're almost there, but before we get to your quote, we need to know about Robert Havemann, an East German chemist and dissident Marxist. From your source, we find that Havemann believed the following:
To be a scientist in MARX’s sense one has to always be prepared for revisionism. De omnibus dubitandum est (everything must be doubted) was MARX’s scientific credo. It goes without saying that even all theories and ideas that come from MARX are among the things that not only may be subject to doubt but must be continually doubted if Marxism is to remain alive....
That is, Havemann argued that questioning everything, including Marx's ideas, was not only proper but would have received approval from the founder himself.
Finally, we come to your quote:
HAVEMANN countered this [i.e., revisionism as any challenge to authority] by turning the condemning word around and presented MARX as a revisionist par excellence.
What this means is that Havemann took the label "revisionist" that the authorities used to condemn their opponents as traitors and made that word a term for legitimate investigators who were working exactly as Marx himself envisioned.