After struggle with this problem for several days, waisting around 1 working days in total, I now can answer my own question.
The behavior "objectClass: olcBdbConfig" causes "ldap_add: Invalid syntax" error simply indicate back_bdb module is not loaded. I guess one may argue an error at importing olcBackend is more user friendly as it clearly reminds not having backend loaded, while others may consider an error at objectClass is more logical to the programming aspect.
Below are off-topic thoughts of mine after this experience.
The document is pretty complete in documenting what the configuration system is made of, but not the process to make use of it. It documents status but not procedures to reach status. I think a well written step-by-step guide and checklist might save a lot of time.
That reminds me of a business case, an engineer is sent to a spot where the supervisor give every information needed to solve the problem, and he went confused and says, yes I have a lot of information now, but what do you want me to /do/? One may took this lack of proper knowledge as lack of motivation, but more frequently it is lack of knowledge. In the very case a complimentary document would help: it contains a list of problem to be solved, a list of steps to verify the problem is solved, and more helpful would be what steps to take to solve the problem. It cover only basic cases so engineer can improvise complicated cases. But such complimentary document is not in the Administrator's guide. Eventually an engineer might find himself not need the complimentary document much, but he needed it in the first place. Another comparison: pupils dump their textbook after learned them but keeps dictionaries. Yet without the text book pupils don't find dictionary much useful. I guess I am in this pupil case and can benefit a lot with text books. I didn't successfully manage find such text books on the Internet in this cn=config case, perhaps they don't exist yet.