Why is the article not necessary here? Shouldn't it be "Questo è il mio"?
Because it's a general statement that "this" is mine, rather than this specific one out of all of them is mine. At least that's my understanding.
My question as well.
Though I was correct, I considered another answer. How would one say "This is me" like during a phone conversation? Questo è mi?
"ciao, sono io" :)