While this moment might be one of the minor complaints that viewers had after seeing Star Wars: The Last Jedi, it was almost certainly the sort of complaint that a geophysicist would notice, thus Mika McKinnon makes a point to call it out because it actually makes perfect sense.
As shown in the GIF included in the tweet, the scene in question takes place when one of the remaining Resistance soldiers grabs a few grains of the surface of the planet Crait and put them in his mouth, discovering that the planet is covered in a fine layer of salt.