It's more of a novel way of seeing what we knew was there, and then some willfully clever writing to make the scientifically routine sound new again. The interstitial space and it's role in carrying debris, waste products, and cells into the lymphatic system is well described, indeed a fundamental premise of physiology in health and disease. Extracellular fluid constitutes about 1/3 of a human's fluid volume. Do you really think we haven't thought about where that 15 liters is?

As was said on the other thread, this is like buying the Hubble Space Telescope, pointing it between Mars and Jupiter and breathlessly reporting a new kind of planet that consists of a lot of rocks spread in an orbital ring.