When a user issues a COMMIT statement, LGWR puts a commit record in the redo log buffer and writes it to disk immediately, along with the transactions redo entries. The corresponding changes to data blocks are deffered until it is more efficient to write them. This is called a fast commit mechanism.

I guess that sentence just mean writing dirty buffer at Checkpoint is more efficient than writing dirty buffer at every commit, and the write of redo entry is the single event that determins the transactions has committed.