It's a big claim, and potentially misleading even if technically true. There could plausibly be a higher accident rate, but lower serious injury and fatality rates. (Due to lower-speed collisions, and reduced likelihood of being knocked into other traffic.)

A quick google found this page [http://www.victoriawalks.org.au/news/1637] created by a walking lobby group, which looks like an attempt to present the strongest case against footpath cycling, but still leaves all the big questions open.

Sidewalks often intersect car trajectories, either due to driveways or street crossings. Pedestrians are relatively easy for drivers to see because they move slowly. Bicycles on the other hand can be moving quite quickly and often believe (often correctly) that they have the right of way. Combine that with poor visibility, typical lack of bicycle lights/reflectors/etc, and unpredictable behavior, and you have a recipe for disaster.