Once again, I see nothing wrong with a 5th kyu giving advice to a "sempai"?
I think that there is subtle but important difference between offering advice and giving feedback.
Everybody should give feedback sempai-kohai, kohai-sempai, what ever it doesn't matter. But what does matter is the nature and tone in which it is done in. My girlfriend a non Aikidoka once said, if people want your advice
then they will ask for it. In the original post, the 5th kyu is not giving feedback they were giving unsolicited advice.
However giving feedback is a slightly different story honest feedback both physically and verbal is, as has been pointed out very critical to development of all parties. It can be done in such as way as that it is non threatening, non patronizing, and not rude. The true intent will be easily by a higher dan grade. When I offer feedback to my sempai, which I often do, I will normally start out by saying something along the lines of, "just to let you know" ... then I would move my arm as you describe. Or If i don't know them I just ask outright, "can I offer you some feedback". I have never had a sempai get annoyed at me for offering to offer feedback.
I also apply a similar approach when giving advice to kohai, I dont just say "do this" "dont do that", (except with absolute beginners) instead something along the lines of "Can I suggest you try doing it this way", "you are going really well, but have thought about...".
You are absolutely right people want and need feedback, but being mindful of the way to go about it is quite a skill, and again another useful skill we can learn/improve on the mat and apply in our every day lives.
Making a clear distinction between advice and feedback
and how we go about doing it is critical. We should always be mindful of the other person state of mind and how they are going to react to what, and how, we say what we say to them.