Yes I get that but in English "This is my book" and "This book is mine" is 100% interchangeable, exact same meaning.
Likewise, in Bahasa "Ini bukuku" and "Buku ini malik saya" are equally interchangeable.
Duolingo should focus on practical language learning and intent/meaning not nit-picking every sentence structure nuance.
It's interchangeable. But here you're learning a language. Duolingo teaches us the vocabulary, words pattern. So, instead of rephrasing the word (which basically has the same meaning), why don't we just follow the translation to avoid confusion.
Mine = milik saya
My (noun) = (noun) saya
If you rephrase it, you get the feeling but would missed the point