I think you are wrong. -na is used for "have". Sina: I don't have, and so on. The negation is in the prefix. Hu- would be "you (do) not". Roughly translated "hujambo" would be: "you no issue(s)". Literally, issues would be mambo, plural of jambo.
Totally agree. In one lesson DL should not have two correct answers and expect us to guess which they want. Mambo and Hujambo mean the same thing although their use my reflect nuanses of context (hujambo would best work if there was a reason for concern)