Well, now i tried it myself, and no, it does not work. The help clearly states that the tree cannot handle multi select choises, the mentioned count is never more than 1 even though many are clearly selected. Bug in my oppion.
And another bug found: SelectionEvent consistently does NOT fire when selected by a moving mouse cursor (as when trying to "drag" the item), even though the item is clearly selected... So you must work-around by a lot of onmouseout etc. HTML DOM events to make it somewhat failsafe.
Might be that MII iBrowser fires the on mouse button up (which is too late when the pointer is not over the item) whereas actually selects the items by mouse down. Clearly an implementation bug IMO.