The author provides nothing to back up this statement. There is NOTHING from Microsoft that says that defragmenting Exchange servers can cause data corruption or issues.
"Because these special-purpose tools safely keep Exchange databases from fragmenting, there is no need for additional third-party tools."
ESEUTIL is used to perform an offline compaction of Exchange datastores. Compacting datastores means that the internal fragmentation of the datastore is eliminated. However, the external datastore - as it exists on the drive - can still be fragmented. If there isn't a large enough piece of contiguous free space on the drive, the new compacted datastore will be created fragmented on the drive.
- Greg/Raxco Software
Microsoft MVP - Windows File Systems
Disclaimer: I work for Raxco Software, the maker of PerfectDisk - a commercial defrag utility, as a systems engineer in the support department.