Is there a contradiction between group commit and non-sync mode?

[pleiadesian]

According to WAL Performance page of RocksDB WiKi, Group Commit is a subsection of Sync Mode. AFAIK, the introduction of group commit aims to increase the write bandwidth of WAL by I/O in sync mode. However, in the non-sync mode, which is the default option and used more commonly, WAL is usually a pure memory operation without any I/O. In the implementation of DBImpl::WriteImpl, the Writer object must go through the JoinBatchGroup process, whether it's sync mode or not. 

Will batching on memory stores introduce more overhead than directly writing WAL to memory?