> The idea is that when someone says "spoonful", they mean "one unit of the thing as you'd normally apportion that thing, using the spoon you'd normally use when interacting with that thing."
> So, one "spoonful of sugar" (or most spices) is a teaspoonful, because you use tea spoons when interacting with sugar in everyday life; while one "spoonful of honey" (or soup stock, or reserved pasta water, or any other thing that you only tend interact with in the context of cooking) is a tablespoonful. (Or actually, a wooden-mixing-spoon-ful, since that's what cooks had on hand to measure with — but as it turns out, wooden mixing spoons usually had the same level liquid capacity as table-spoons. The heaping measures weren't equivalent between wooden mixing spoons and tablespoons, though, and this has caused much numerical error when modernizing old recipes!)
The answer is that a "spoonful" is not precisely defined. "Tablespoon" and "teaspoon" are defined measurements, but "spoonful" isn't.
The answer from another comment is that the author meant "tablespoon."