Despite using setChartExpandCollapse(true) the HTML looks like this:
<div class="orgchart noncollapsable" ...>
The issue can also be observed in the Demo (Drag and Drop Tab): https://addonsv14.flowingcode.com/orgchart
EDIT:
Using setChartDirection(ChartDirectionEnum.TOP_TO_BOTTOM.getAbreviation()) allows collapsing most nodes, except for one last node below the root node. Also the collapse / expand arrows don't show up at all, so the end user wouldn't know, that the nodes are collapsable.