From a purely technical point-of-view, too long answers are answers containing more than 30,000 characters (meaning around 5000 English words!). Yes they are bad things and are actually truncated according to this Meta SE discussion.
But this remains an exceptional situation.
When one flags a question as too broad, the issue is not really that the answer would be long, there are good long answers (the canonical ones being the best examples), but the issue would be that the answer will also be too broad.
The "format" mentioned in your quoted text implies a precise question being associated to precise answers. The goal of this is that SE answers can be used as direct reference by other people: Your question has already been asked? SE serves you directly the most sensible answers. It becomes a real time-saver.
When the question is too broad, whether because it not precise enough or actually include several "sub-questions", then the answer will have to cover all possibles situations or answer every sub-questions. Such answers becomes less usable for the purpose of this site: Here are a bunch of information around your subject, maybe your answer is in-there, maybe not, we let you sort this out...
An interesting reading on the subject can be found on Meta SE, I would tend to summarize all-this by saying that a SE answer should remained focused and avoid digressions and irrelevant informations, all this being impossible when trying to answer an already "too broad" question.