They have a turing-complete CPU. They have memory and storage. They have input and output devices.
That pretty much sets them as general purpose computers.
Oh, that's not their intended purpose - not what they are marketed at. Overcoming the lockdown may take some work. But you CAN run arbitrary computation on them, even with the lockdown.
Even on IPad, where they paid close attention to disable general computing to a degree where Commodore 64 emulator was banned because it runs Commodore BASIC, you can still load a page that contains:
<textarea id=x></textarea> <input type="button" onclick="eval(document.getElementById('x').value)"/>
Note, by the same virtue, a Postscript printer is also a general purpose computer. But yeah, using the right Postscript, people were playing chess against Postscript printer/scanner devices - the printer would print the chessboard with the pieces, the player would draw the move with an arrow, then scan it in, the printer would recognize the move, calculate a response and print it out. So, yeah, that's general purpose computers for you.