According to Wiktionary, "mambo" means 'things', 'affairs', 'issues', so when you greet someone with "mambo" you're literally asking them "[How are the] things?" (not too different from English "How are you?"). That must be the reason why it is written as a question.
By the way, the greeting "jambo" (which according to Duolingo is only used with tourists because foreigners believe it to be a greeting) is actually the singular form of "mambo" (that is, 'thing', 'issue').
Haha! Duolingo says that as if the only Swahili is Kenyan Swahili. I personally know (as in go to their homes and eat food with, teach their kids to read, and help the women have babies,) a large number of African refugees who speak aproximately 5 different dialects of Swahili, and I promise you, apart from "Habari Yako", "Jambo" IS a very common, very legit greeting in Congo and Ugandan Swahili. They answer the phone Jambo in Congo and Ugandan Swahili, and so do some of the Tanzanians (and not just to me, but to other Africans, too).
So... I guess if you're prepping for a trip to Kenya that might be true, I only know two Kenyan Swahili speakers, and they do kind of giggle when I greet them that way. You can see the look of "Oh how cute! The teacher is trying!" on their faces! HAHA! I also made the mistake of greeting Congo/Uganda/Rwandan Swahili speakers with "Habari Ghani", which is apparently distinct to Tanzanian Swahili just makes everyone else laugh and correct you.
Mistaking a greeting isn't as bad as saying the totally wrong, but similar-sounding word, though. Africans can't help themselves, I think. When I mess up, which I frequently do, they just bust up laughing. Gotta have a thick skin to learn Swahili from Africans. They are so polite and respectful, until you say "now concubine 3 (suria tatu)" instead of "now paragraph 3 (sasa aya tatu)". Then they're practically falling on the floor. (Can you blame them?)
Oddly, "Mambo", at least in Tanzania has rules that change depending on the culture of the speaker. My Tanzanian friends greet thier elders or people in higher positions strictly with "Shikamoo". To say "Mambo" to an elder or an official has some serious consequences. The kids all laugh when I say "Mambo Vipi", or "Mambo", or just "Vipi" (Tanzanians will often just say Vipi to peers, like saying "'Sup" to your friends in America) because to them I'm an old woman! I know this, which is why I do it. :)
Anyway, Swahili is so broken and divided that what is a hard/fast rule in one place isn't so in another, so when you have a program like this trying to teach uniformity, some things are bound to be true to the dialect of the app developers but untrue globally. (I'm convinced that with the number of African refugees migrating to America in recent years, it won't be long before we have a new dialect: American Swahili.) Nevertheless, everyone says my Swahili is improving much faster now, and I know this is true because there is much less laughter and falling about when I talk now than there was before duolingo. So, I'll take it!
One time when trying to set up a time to come back I accidently said "Nitarudi Saa 'Kuma' (instead of Kumi) na Mbili" which is pretty vulgar and I got some funny looks, followed by an uproar of laughter. I guess you're not really learning a language with out swearing a few times on accident. Or atleast saying offensive/innapropriate things!
"Hi Emilian" is a good translation as it is informal, just as "Mambo" is. We're getting typical greetings here but I've heard many more such as "Habari za siku nyingi" "Vipi mambo?", "Vipi" and "Habari ya saa hizi?" Getting to know the language does not require us to know many greetings.