Hi Will,
Not exactly. Although Codename One (CN1) apps are written in Java, the framework makes use of its own set of APIs etc, abstracting away from platform specifics.
Apps are then built/compiled on the Codename One servers. These servers return the installable application, depending on what platform you selected for the specific build.
So, for example, if I right-click on my project in Netbeans, then select 'Send Windows Phone build', the Codename One build server will compile my Java code, and send back an installable app (in this case a .XAP file).
Cheers