This has been a developing understanding about the giant planets over the past few years. This new result derived from Saturn’s rings just provides another line of evidence that quantifies it better for Saturn. Jupiter also has a fuzzy core and it is almost certain the ice giants, Uranus & Neptune, have fuzzy cores as well.
The old models of distinct differentiated layers are being replaced with mixed layers and density gradients. Part of this comes from studying the behavior of planetary materials (silicates, water, iron) under different pressure and temperature regimes not previously studied. These “equation of state” studies improve interior composition models.
One very interesting paper last year that came out by Vazan et al found that for Earth mass and larger planets rock and ice are >99% miscible in the interior. This means that water-rich super-Earth to Neptune mass planets likely have a fully mixed rock-Ice interior rather than a distinct rock core overlain by ice and liquid ocean layers. Vazan’s team did find that for planets the size of the rock-ice Satellite planets in the Solar System (Europa, Titan etc) rock and ice are not highly miscible so the interior models we have for the outer ice-rich Satellite planets and Dwarf planets are still acceptable models.
Here is a recent paper by Wahl et al. 2017 that presents a fuzzy core model for Jupiter. Look at figure 1 to see the more complex structure compared to older models:
Here is the arXiv posting on the model developed from Saturn’s rings for those that want the details: