Yes, you've come to the correct conclusion.
As of version 2.2.0.Final, Errai gets all the information it needs about client-side types from the GWT TypeOracle at rebind time. The ErraiApp.properties file (note: begins with uppercase E) does hold optional configuration info (such as declaring portable types that you can't annotate with @Portable), but it is essential as a marker for our runtime classpath scanning in the server-side code.
We can't guarantee this will always be the case for pure client-side apps in the future, so I'd still recommend you do put one in the root of each .jar.
Thanks, Jonathan. That's good information.