As penguin indicates, "dash" is not
limited to liquid ingredients -- although it is more applicable to them than "pinch" would be, which may be why you often hear it in the context of liquids..
1) : a small usually distinctive addition a dash of salt a dash of humor
2) b. A small amount of an added ingredient:a dash of sherry.
3) A dash of something is a small quantity of it which you add when you are preparing food or mixing a drink.
[Oxford does define it as a small quantity of liquid that you add to something, but then gives a second related definition in which it relates to "A small amount of a quality that adds piquancy or distinctiveness to something else.
‘a casual atmosphere with a dash of sophistication’"
Frankly, neither "dash" nor "pinch" are very precise terms, and I wouldn't make too much of a big deal in distinguishing between their use in a recipe. To me, a "dash" of X or a "pinch" of X really means just "season to taste with X."