Until Ext JS 4.1, the signature of the event Ext.data.TreeStore.beforeappend is [ this, node, eOptions ] where you can use this as the parent of node. So, 3 arguments.
Now trying 4.20.265 beta, the signature has changed to [ [parent, node], eOptions] (2 arguments, where the first is an array). I would suggest that something there is a tiny change (typo?) when applying arguments to the event callbacks..?
Either this is a documentation bug or an internal "bug".
Do you need more information about this issue? Personally, I think that should be sufficient.