...is the scale of the system they observed the effect in. Since the second law is a statistical law, in small-number systems you'd naturally expect to see it break down once in a while. But this particular experiment featured latex beads a few microns in size, suspended in water. Relatively speaking, these are huge objects, comprised as they are of tens of billions of atoms. The sorts of naive counting argument that have indeed been around since the 19th century would say that seeing any kind of fluctuation on this size scale would be extremely unlikely. You need some substantially fancier math to get a more accurate estimate, and this is apparently what was invented around 10 years ago. Having experimental confirmation of the theory is very important too, because it may mean that cell biologists will have to update their models: cells are on the same size scale as the objects the physicists studied. Mesoscale physics is on a much bigger length scale than we thought!