You can use QgsComposerFrame.multiFrame():
table_frame = printcomposer['QgsComposition'].getComposerItemById('table')
table = tabletable_frame.multiFrame()
Multiframe items like tables and HTML items can have their contents spread over multiple "frames". The getComposerItemById method is returning a frame for a table, but not the table itself. Calling the multiFrame() method returns the frame's parent QgsComposerMultiFrame object, which in this case will be a QgsComposerAttributeTableV2.