Supernova's emit x-ray bursts and a wave of non-photon particles. If the particle wave hits a thick enough nebula, Physicsy Things happen that will polarise the x-rays. This process can only cause partial polarisation (60% ish) and usually well below that.

They found a supernova with x-rays polarized nearly to the limit, which would only be possible if the particles were a fluid flowing nearly perfectly with almost no turbulence. That would be, to put it mildly, a near trick.