I believe that you are asking about the validity of critics pointing out that Chinua Achebe incorporates elements of Nigeria's rich oral storytelling tradition into his seminal debut novel Things Fall Apart. If that's the case, critics are absolutely justified in noting the importance of oral traditions in Achebe's canon as a whole. Achebe himself consciously presents components of oral tradition in order to better convey the juxtaposition of tradition-driven Okonkwo and his inability to reconcile the rapid changes associated with Western colonialism.
As far as a concrete example, the most striking aspect of an oral heritage present in Things Fall Apart is the importance of snakes in the Igbo tradition. The snakes are revered in the Igbo tradition, and Achebe consciously uses this image in order to faithfully write an English language novel that remains distinctly Nigerian.