This sort of exchange makes me yearn for days before Google, when only people who knew something would talk about it 
What I am saying is that Iman is an Arabic word. Anyone who knows anything at all about Arabic, Arabs, or Islam would know this. This word has gone into the languages of every Muslim nation, not only Swahili, along with other religion-related words, like heaven, hell, "Allah" (= God in Arabic) etc. They are no more Swahili than they are, Turkish, Albanian, or Persian.
Some early baby name website has singled out Swahili, native language of a mere 5 mn people, for Arabic names that have been adopted in all other Muslim countries - names of Mohammad's wives, female members of his family, etc. You may have noticed how much they all copy from each other and make the same mistakes (ex: They all say "Cari" is a girl's name in Turkish that means "flows like water". It means something totally different, is not a name, and C isn't even pronounced K in Turkish.)
So, when someone says "Oh this word means this in Swahili", you can usually bet that they have read it on some baby name website on the internet.