To sketch something is to make a rough outline of something, in the way that artists often create a sketch of something in pencil before painting it in oils. I read "barely sketched" here to mean that the person has vaguely defined the task of unravelling the political "ball of yarn" (skein) once he utters the words "everything is political".
In other words, to say "everything is political" is to barely scratch the surface of the complicated world of politics.
