Yes, the point of Opera Mini is to provide web browsing to devices too underpowered to have a full-blown web browser. It's written in Java because most of the phones in the class Opera is targeting have some form of Java VM. What effectively happens is most of the Opera rendering engine is on Opera's server, the Opera Mini client asks the server for a page, the server downloads and partially formats the page, and sends the result to the client, which has a much easier time displaying the partially-digested (so to speak) page.

Since the 700/N800 already have full-blown Opera onboard, rendering engine and all, there's not much sense in having Opera Mini on there too.