So, I'm confused, because I just read your instructions on using "dir" at the end of the word, and I understood that it's only used for big, important, changeless sorts of things, or for uncertainty, or.... Why is it used here?
I'm not sure either but I've heard it also expresses uncertainty. Like : A: kedim hiç bir şey yimiyor . B: Aç değildir. İt's probably not hungry. Maybe this sentence is the same case. İ'm not a native speaker . I'm learning as well but there was a complete topic about "dir" on the website. İt could be helpful : https://forum.duolingo.com/comment/8649151/Grammar-Suffix-dir