Microsoft does something cool for once and it's an accident?
Probably the author observed that reading different files from disk simultaneously was slower, until the copy process catches up to read the same files.
System RAM or Drive cache REALLY helped, so yes, the lagging copy would drag down the leading one until they both caught to within that buffer and things zipped along.
I've observed this same behavior on modern operating systems with big rsync jobs and spinning drives. If you have lots of small files then reading the metadata and directory structure takes a while but fits in the cache, so a second rsync will catch up very quickly to the first.