You will notice that the former figure S1 (in the Supplemental Information) is actually called a Supporting Figure. I guess this helps distinguish it from the completely-different-and-in-no-way-to-be-confused Supplementary Figure S1.
This would possibly make some sort of sense if the main body of the paper distinguished between the two different types of Figure S1. Except the paper mentions 'Supplemental Figure S1' twice (not even 'Supplementary Figure S1) and doesn't mention Supporting Figure S1 at all (or any supporting figures for that matter)!
What does all of this mean? It means that Supplementary Material is a bit like the glove compartment in your car: a great place to stick all sorts of stuff that will possibly never be seen again. Maybe we need better reviewer guidelines to stop this sort of confusion happening?