I was also not sure why they did not say this, as in previous German classes I've never heard "mein Name ist..." taught.
My best guess is that Duolingo is trying to teach a simpler way of introducing one's self before teaching the more ubiquitous "ich heiße" later. Either that, or there is actually a contextual difference between saying "I am _" and "My name is _", similar to the slight different there is in English.
What's the difference between introducing yourself with "Mein Name ist [x]" and "Ich heiße [x]"?
But there's still not answer to why "mein Name ist" is used rather than "ich heisse." I was taught "mein Name ist" is only used by lazy English-speakers because it's a direct translation of what they would say and isn't used at all, ever, by Germans. And in the German introductions I've been a party to, the person has always said "ich heisse." (Actually, most people in informal situations just say their name, or "ich bin...." but if they use "my name is...." they say "ich heisse….")
No, "Hallo, mein Name ist Duo" means "Hello, my name is Duo".
I think you're talking about "Ich heiße Duo". While "heiß" (adjective) does in fact mean "hot", "heißen" (verb) means "to be called". Therefore, "Ich heiße Duo" means "I am called Duo".