Would you consider the filled recipient to be truly filled? there are 2 pixels flicking inside the cup. Then again, what does it mean for something to be truly filled?
You could say tha full cup means that all of the matter that is surrounded by the cup, everything that is inside the cup but if we answer this way then we have to explain what does "inside" the cup even mean. If you were to talk about a box (as one usually does) then it's pretty easy to understand what part is the inside and what part is the outside but it's different when talking about a cup, a glass or a cup.
You could take the "invisible wall" approach where you take everything that is encompassed inside the walls of the cup, the bottom and an invisible wall tangent to the y axis that starts when the walls stop. This is an acceptable way to consider it.
So then, now that we've defined what the inside of a cup is we would assume that a full cup is when all of its insides are filled or in other words their insides contain matter that of which is not air. You could also say that a full cup is just cup that cannot contain any more material otherwise it would spill. But then, none of these definitions actually explains why a glass that contains a mayority of water and a bit of air clasifies as a glass full of water.
Language is weird, man 🚬