How long is a piece of...
Common-or-garden PKZIP can reduce a typical .xls by 50% or more. 7-zip might do even better.
For .vmdk files, compressibility will depend on whether all space is allocated at creation, or not. If yes, then there will be acres of blank space which can be very easily compressed. Now, if they'd taken already-compressed .jpg, .mp3 or .avi files into account, the results might look less promising.
For years, backup software and hardware vendors have made unrealistic claims as to media capacity, through quoting hypothetical compressed values instead of actual figures. This leads buyers who are unaware of this hype into specifying backup solutions which are inadequate for the task in hand, for example a "500GB" tape drive typically will NOT backup a full 500GB disk. Sometimes, not even a half-full disk. Consequences are eventual failure of automated backups to run properly, and the need to replace costly but under-spec'd backup equipment ahead of life-expectancy.
This claim might be genuine, and I'm prepared to give it the benefit of the doubt. But, I'm wise to the backup industry's reputation for hype.