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').
How are you Emilian ? It is a proper form, but " what's up Emilian ? " also works. These two forms have a questionmark. " Mambo/ jambo" means also - hello / hi .
"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.
In English, "hello Emilian" is not a question. Is it a question in Swahili?
I put hi Emilian for the translation, and it said that I used the wrong word, and the right one is hello Emilian.