Using crashplan to backup a time machine disk
Solution 1:
Backups are important, right? So I'd tend to take Crashplan at their word, rather than try and prove them wrong.
However, if I were going to try and use Crashplan with TimeMachine, I'd look into backing up time machine to a sparse image, and then using crashplan to backup the sparse image. TimeMachine automatically backs up to a sparse image when doing backups to a network location. The sparse image itself is just directory containing basic files, these are used to provide the filesystem features, like hard links, that TimeMachine requires.
Whatever you do, remember to test the backup and restore process before you put any faith in it.
Solution 2:
Do not attempt to prove crashplan wrong. Many bad things can happen.
Use the new Time Machine (since Mountain Lion) instead.
Problem solved.