I don't think that there is any confusion about whether the question is off-topic according to site guidelines, since the quote you provide states clearly that it is. But within the community the application of those guidelines may vary. Even one of our venerable moderators would seem to agree with you in regards to this question.
My instinct is to agree as well; I didn't vote to close this question, and have a hard time with casting votes to close similar questions. It does seem to be a perfectly reasonable question.
But, the overarching goal of the site is to create a searchable repository of information that will be useful to future visitors, and questions like "What Musical Instrument Is This?" don't seem to fall into this category. It seems unlikely that a future visitor with the same question would be able to find the answer without blindly checking all of the instrument identification questions, so not very useful for future visitors. Do we want to allow similar questions that show an image of an effects pedal or similar device asking for identification?
We have a similar situation with notation identification questions. These don't seem to be explicitly forbidden in the help pages, but I think that it is implied; these questions also seem unlikely to be useful for future visitors. Yet the community seems to accept notation identification questions.
I suppose I have mixed feelings about this. Should usefulness to the asker outweigh the site mission and usefulness to future visitors?
This seems like a can of worms, and I'm not sure that rewording the help pages will solve the problem. My personal approach is just to abstain from voting to close on questions like this when I have such mixed feelings.