Don´t quote me on this, but I think that "hui" means to meet as a verb, but in Hawaiian many nouns can be used as verbs and vice versa. "Ana" following a verb means a single event, therefore one single time these people met is a meeting and that is why there is ana behind the hui, to indicate the single event of meeting.
However, this is just my deduction, so I am absolutely not sure about this and anyone can feel free to correct me.
Good question, Ian. I learned that a nominalized verb turns it into "the act of" instead of a generic descriptor noun. However, Hawaiian is fluid and there are multiple possibilities, and the course is still in beta just as an aside. I myself am wondering why they are not teaching hālāwai instead actually.