I have one of these instruments and they're not in a diatonic major scale. Draw your own conclusions.

It's just a bunch of tubes and reeds, you can presumably get one tuned to whatever you like. Variants of the instrument are found all over China and South-East Asia and they don't all have the same scale.

I've got my diatonic autoharp tuned in hijaz at the moment. The designer certainly didn't think of that.