I’m trying to find a way to refute this, but I cannot.

It doesn’t need to be fully watertight. The rate of water passing into the dry area only needs to be lower than the rate you can pump it out.

I’ve just come from Sync, and this is the feature I miss most

Its a misconception that fluid means liquid. A fluid is a substance that has no fixed geometry, and conforms to the environment around it. If you adjust for time, you could make an argument that ice is a fluid, as glaciers flow almost like a river. Steam is definitely a fluid, as are all liquids and gases.