The really important part of Achebe's statement here is the phrase "of his time." Okonkwo's culture is characterized by strict traditions of age, gender, and culture. Okonkwo's "time" tragically ends just before he does—the fateful moment when none of the other villagers stand with him to fight against the European soldiers.
To be "great" is a subjective idea, based on the morals and values of the culture and time period where a person exists. For Okonkwo in traditional Igbo culture, he was truly considered to have excelled in all areas a man could excel.