Strongly disagreed.
I believe you are correct.
The original line is: あ もう ぬるい (a, mou nurui). Mou meaning "already", nurui meaning "lukewarm".
I assumed they were drinking hot tea and that it had become tepid. Largely because I drink a lot of hot tea, even on very hot summer days, and I hate when it gets tepid. It never occurred to me that "lukewarm" could also refer to a cold drink becoming too warm, and that this would be just as unpleasant. However, the condensation on the glass does indeed suggest that this is what happened.
Thank you for pointing out the mistake. As the translator for this project, I apologize for the error. That line should be, "Yuck, it's all warm."