Really interesting way to make fdm watertight and stronger

[curious aardvark]

Cool technique. Casting sand like Delft Clay or Petrobond would probably work for this. Casting sand is extremely fine and can take very high temperatures.

Yes sand would work - up to a point.
bear in mind that the sand will get embedded in the plastic. And getting rid of sand is really hard. You can't 'sand' it off as it's as hard as the sanding paper.
Silicon dioxide is tough stuff.

The really clever thing about using salt is that it is water soluble.
So all you do to remove the embedded salt is to rinse or soak it off.
The other clever thing about salt is that you can grind it to different 'grit' sizes yourself.
If you only have coarse sand - you cannot grind that smaller yourself.

If you wanted to make an abrasive block or some custom shaped 'files' then you could use sand.
Of just pack the area round the print with tungsten carbide dust. And use salt or sand for the bulk.

But for non-file type uses - you do need to use salt.