There are two places where items get added to reorderable_items-- here for a single xblock, and in a xblock mixin for Studio support. In the second case, multiple items may get added to the list, as the xblock being rendered may have children. Therefore it needs to be a set in general.

It's ugly that we have to populate the set in two places, but when just a single leaf xblock is rendered, the mixin has no way of telling if the parent of the xblock supported reordering.