Let’s do some logic:

I assume the answer is “no” – at tempo = 60 this is not really rapid.

Again my guess is “no”, neither the notation or the tempo would qualify this as a tremolo.

Again the logical conclusion is “no”.

So why do we sometimes encounter this notation shown above? I would genuinely love to know why, and where this tradition comes from, and what kind of logic it is based on.

Please, convince me!

I will tell you my objections in further detail, for those who are similarly pendantic-minded like me. In essence, this is a term for harp players. A little appropriation among musical terms is fine, but I think this extrapolation meaning “any movement between harmonics regardless of context” is misleading, which brings me to my next point:

Sometimes a passage can be played with a change in harmonics AND bisbigliando/timbre trill, therefore it makes more sense to be specific.

So if you don’t really want a timbral trill, what would you write in the passage shown above? The good news is: nothing! You don’t need a descriptive word or any notational elaborations, because the notation speaks for itself. If you want a descriptive word, perhaps use “sotto voce”, “lontano”, “quasi niente” or something to that effect.