Re: Am I really missing something?
"Or is this pretty basic schoolboy physics?"
Yes, you are missing something: mainly that your schoolboy demo didn't involve a careful experimental design and setup, comprehensive results/image taking, and proper analysis of the results. And that your "thick liquid" is under no compulsion to behave like the granular materials tested here.
Why on earth do you think a rewrite of a press release and a simple analogy aimed at the wider public constitutes the entirely of this research work? Let alone what will make up the content of the PRL?
Why not visit http://journals.aps.org/prl/ and look at a couple of the articles there that are open access, and test your schoolboy physics against those? It might give you an idea of the level of technical detail this kind of press-release science jornalism leaves out . And don't forget, since PRL has a page limit, the writeup there is often somewhat abbreviated.