Arranger keyboards is for me musical instruments. 🎹🎵
It's possible to make it sound your own way, i.e by the use of aftertoutch, expresion pedal and more.
Modern arrangers has also got more authentic sound like the original instruments as it is used very natural samples.
As long as we play it fysically to create and express music, my conclusion is: It's a Musical Instrument, no doubt.
But it also allow you to use it as a pure 'playback machine'.
All that sudden it can turn into a midifileplayer or even a 'karaoke machine'.
If so, maybe it's your or other human voices that is the real Musical Instrument. 👨‍🎤👩‍🎤