Certainly in the rough sense of 'Turing machine' a smartphone is clearly a computer. And, even in other senses, I can connect a keyboard to my S5, open up Termux, run ffmpeg, open up Emacs --- all on the smartphone itself, not through ssh or anything. But it's hardly as productive as a regular laptop or desktop, of course, but that's largely due to screen size, lack of keyboard (or lacklustre keyboard), and weakness of processing capacity compared to a desktop/laptop.