I just got an answer from a native speaker on the forum: https://www.duolingo.com/comment/27575859
What I understood is that from 1.000 to 9.999, you will use "elfu XXX".
From 10.000 to 99.999 on, you'll use "XXX elfu".
Thus 18.000 is "kumi na nane elfu"
And 10.008 is "kumi elfu na nane"
And even if some people say "elfu kumi na nane" (like the answer on this exercise), it's not really correct because it can lead to ambiguity.
This is confusing, so there's a solution to it: Every time the tens of thousands are not followed by any other number, the word "elfu" will be placed after the number that quantifies it. Thus, "elfu kumi na nane" is one thousand and eighteen, and "kumi na nane elfu" is eighteen thousand. The translation is incorrect.
This is incorrect, the number should always be led by 'elfu.' As for "elfu kumi na nane" this is not one thousand eighteen, but eighteen thousand, the translation is correct. My guess, to distinguish between eighteen thousand and ten thousand eighteen, is that 'kumi' will simply be repeated, resulting in "Elfu kumi na kumi na nane. To say one thousand and eighteen you say "elfu moja na kumi na nane," Where elfu moja is one thousand, kumi is ten, and nane is eight, equalling one thousand and eighteen.