There is a strong argument against Achebe's criticism. Conrad's novel includes textual commentary related to race, nationality, and colonialism, but his book is ultimately about the nature of morality and the limits of the individual to think andbeactually individual.
The subject of racism is not really treated by Conrad as a theme in Heart of Darkness as much as it is simply shown to be the prevailing attitude of the day.
This is a book about one man's attempt (Kurtz) to generate an original moral code, to separate himself from the moral considerations of both the civilization from which he comes and the tribal culture in which Marlow finds him in the end.
When considering the central concerns of the narrative, the clear assessment suggests that morality and an existential angst regarding the nature of evil anchors the novel. Most of the novel's themes are related to these ideas and the binary concepts that they imply: sanity vs. insanity, society vs. wildness, etc.
Neither Kurtz nor Marlow, the novels main figures, are concerned with race or nationality. They are concerned with far more existential issues, resulting from circumstances as well as a particular will:
loneliness and alienation have taken their greatest toll on Kurtz, who, cut off from all humanizing influence, has forfeited the restraints of reason and conscience and given free rein to his most base and brutal instincts.