I don’t understand this use of terminology. Is every other computer not a device? 
I suppose its because the word device has connotations of smallness. A device or gadget is something small, a machine or appliance is something larger. Phones are small, so people call them devices.
“n. A thing made or adapted for a particular purpose, esp. a mechanical or electronic contrivance : a measuring device.” Nominally an iPhone is a phone, therefore a device, whereas a desktop/laptop computer is a general-purpose tool. Obviously there is, technically, a huge (almost complete) amount of overlap, but that’s language and convention for you.
–Eric