Very cheeky title indeed…
It’s just a term to describe a unit of measure, as a volume. Doesn’t mean it has only to do with measuring liquids. One fluid ounce is approx 28 (UK) or 30 (US) mL, depending if you mean UK fluid ounce or US fluid ounce.
So, it doesn’t matter if you measure a powder or liquid…a fluid ounce is a fluid ounce. Just make sure you know which one they mean…UK or US version (or just live with the few mL of discrepancy).
Also, when baking, it is better to use weights for dry ingredients, as opposed to volumetric measures. That’s because powders can expand or pack down into the measuring container. If you put a powder in a container, and tap the side, you will notice the powder level will lower. Typically, if a recipe uses volumetric measures for powders, don’t tap the container. Take a flat edge, like a knife, shave off the top to make the dry ingredient flat at the top, and that’s it. If you want more precise measure, find a recipe that uses weights of dry ingredients instead.
Hope this helps.