The only officially supported backup method with OpenLDAP is slapcat. Everything else, you do at your own risk.
The admin guide disagrees with you. Chapter 19 describes incremental backup by copying first the entire DB, then backing up further DB logs.
Personally, I think that article is wrong, as I believe you would at a minimum require id2entry, dn2id, and all of the current BDB log files. I also would not trust any FAQ entry that references LDBM, as that indicates that it is of significant age.
Agreed. Copying anything without the logs sounds broken. -- Hallvard