The Amiga is an open platform, isn't it? Or am I completely misunderstanding that term? While a console usually are completely closed, and you can't run stuff on it not approved by the maker.
If I do understand that term, I assume a computer is an open platform, while a console is a closed one. Custom hardware or not. And no, I don't think the mouse and keyboard is enough to call it a computer - in that case, you could call the X360 and PS3 a computer as well.