The import appears to be failing because their implementation of importNode is calling getDOMImplementation(), and our DOM proxy doesn't currently support that operation.
We can add it. It ought to be relatively low risk, but I'm not sure whether it's critical enough to add during he D13 stable period. Workaround, in the meantime, would be to use one of the many user-level DOM-tree cloners written before importNode() was added to the DOM. You could even swipe the logic out of Xerces' implementation of cloneNode. I'll log this into Bugzilla so we don't forget it, but I'm not sure what priority to give it.
