"Optimistic locking" is a bad choice of words because no locking is optimistic.
In the well-known access method described here, the writers access the shared data with mutual exclusion, i.e. they use locking.
The readers use no locking, but they access the shared data concurrently and optimistically, hoping that the access will succeed at the first attempt.
When the readers are unlucky, they must retry the access.
So there is locking used by writers for mutual exclusion and there is optimistic access used by the readers.
There is no "optimistic locking", which is a contradiction in terms (locking is pessimistic).
In general, there are only 3 methods for accessing shared data: mutual exclusion (a.k.a. pessimistic access), where locking forces the accesses to be sequential, and 2 methods where accesses may be concurrent, optimistic access (a.k.a. lock-free), where retries may be necessary, and dynamic partitioning of the shared data (typically used for shared arrays or for shared buffers/queues), where neither locking nor retries are needed.
The method described here for accessing B-trees employs a combination of all 3 methods, because the release of the locks at higher levels is a consequence of restricting the future accesses to only a part of the shared data, i.e. the writers that access the shared B-tree start by accessing sequentially the root, but then they partition the tree between themselves, so the next accesses that fall in distinct subtrees may proceed concurrently.