Sure, but I could similarly say that most water in any given terrestrial environment does not flow through rivers, but is transported through tiny pores in the rocks, soil, etc. It's a true statement, but does not change the fact that there are rivers in the terrestrial environment. There are underground rivers in karst regions. Cave divers swim through them all the time, for miles at a time. No amount of linguistic gymnastics is going to change that. I'm conducting a water quality study right now in north Alabama in a watershed which is entirely drained by an underground stream network. I've walked/swam the entire main trunk. It's there. There's simply no way around it.
Check h t t p://cavesurvey.com/long_underwater_caves.htm for some underground stream lengths. Keep in mind, also, that underground streams are very poorly known, and some of these could run for many more miles.
Now, we can quibble about the volume, flow rate, length, width, or depth of what constitutes a "river," but frankly that's really a tomayto/tomahto argument, and pretty nonproductive.