The inRenderFlowThread bit on RenderObject assumes that all the objects belonging to the flow have the containing block inside the RenderFlowThreadObject. This is not true in the case of RenderMultiColumnBlocks. See https://bugs.webkit.org/show_bug.cgi?id=110624 that removes the positioned property from the flow thread.
This change is no longer required after the introduction of the FlowThreadState flag on RenderObject. See https://bugs.webkit.org/show_bug.cgi?id=111110