The comparison is not exactly in its place. Liechtensteiner believes that the answer is found on what happened before so as to understand the present state of the glass. This is a nuanced way of looking at it, and it's more accurate, but I did say to him that both can be said actually, if one would not be specific in this.
I am not a linguist, but in terms of semantics you are still wrong I think.
Following the original post, if you had taken water out, it's not that it's half empty, but rather that it is half emptied
Regardless, you are missing the point of the exercise. My rain example was the closest thing I could come up with that was similar to the cup example, and fair enough it's not a perfect comparison, but I am still making the same point.
If it's raining, and I am in a good mood, I would still see the rest of my day as something to look forward to.
If it's raining, and I am in a bad mood, I would likely be grumpier than usual and will not be looking forward to the rest of my day.
Assume in both examples that I am not a footballer or farmer, just an ordinary citizen.
My outlook on both days in which it's raining changes depending on my mood. As would be in the case of the glass, I would say it's half empty or half full depending on my mood.