If it is, then do people at Citizendium suspect whether a large number of those vandals are Wikipedia fanatics who are hammering Citizendium because they perceive some rivalry between the two?
It is unlikely to be Wikipedia fanatics. More likely wikipedia vandels who have found a new target. This happens to most wiki projects with any profile.
A person who claimed to be the vandal in question claimed to be a Wikipedia contributor, specifically saying that he contributed clean-up scripts and stuff to the project. I can't say that that's true, but a fair bit of his story appears to line up properly. Of course, if he is a vandal, he is by definition untrustworthy, but here's
his claim.