Questions should ideally be well written. But believing that a question was posed poorly is not enough and is a very gray area. One should know it. (if a person is not understanding a question, then this may have two reasons, only one of them is that the question is unclear, the other reason occasionally occurs as well)
The crux with that linked question was that voters may vote something for being off-topic while it is not. The problem may not always be in the question but it can also be in the person(s) voting for the question.
It is not like voters never make mistakes. It becomes a semantic discussion whether a mistake like in the link, where somebody was convinced about the vote, is to be called frivolous or not.
The particular question that I linked to is and was very clear (it even explicitly stated the issue that they have “why is P(Y=k|X=15,B) = P(Y=k|B)?” ), or at least clear enough, and there have been reasonable answers (one of them is mine) that show it was not too difficult to provide an answer.
If these type of questions (a person studying a book and encountering a remarkable feature and wondering how that works so he places it on a Q&A site) are not possible then we can just as well close the website.
Note that in this particular question the person asking the question may have been unclear or vague but that is because the person was confused about the topic. The point of the question is to take away that confusion. This follows the saying “there is no such thing as a stupid question” (on SE/SO many people believe this is not so).
I could answer the question because I understand the purpose of the exercise that the OP made an inquiry about (some introduction to beta-binomial distribution by having a mixture of two binomial distributions), and at the same time understand why the OP could be confused about it (the exercise is a contrived example, and may easily confuse a person that is overthinking the reasons for it).
The most important thing that the question needed fixing was a better title and improvement of the image (conversion to text). But that is far from a good reason to down vote or close vote. Such reception is making people feel very unwelcome (and it is unbelievable that SO/SE company is fighting their status as being unwelcome, while having these unwelcoming tools in place).
I’d say we could introduce an option to mark questions as ‘requiring updates’ or ‘need improvement’ and make ‘close voting’ something that is reserved for questions that are really problematic and should not be answered (because of the risk that answers will not make sense).