Yes, you’re right. It’s a very slippery slope. If samplers are instruments, what’s next? Midi clock generators? Soundproofing walls?
If is capable of making sounds in a reproduce able musical key it is a musical instrument. It’s the same thing that makes a Harpsichord or a Didgeridoo and instrument.
A plastic bucket isn’t a musical instrument, but put a bunch of them together in different shapes and sizes and materials, and yes, that’s a musical instrument. In other words, the sum of all the parts of the buckets adds up to an instrument. You may be using it as an arranger, but that’s just one of the things it is.