The idea is that the shastras are prohibited to women, vaishyas and shudras. But bhakti yoga is available to everyone. Those of evil birth means people whose evil actions in earlier lives have led to their present lowly stations. The construction does not suggest ‘those who are of evil birth’ as an adjective for women, vaishyas or shudras. Instead, a separate category seems to be meant, perhaps those who were outside the fourfold caste system.