It is mentioned in the link you referenced:
Compatible with non-Maven Android Library Projects!
The generated .apklib file will have the layout of a standard Android/Eclipse library project. This means that regardless of your Maven layout, the layout inside the .apklib file will be that source code is in "src/" instead of "src/main/java/", but still interpreted correctly when used in an Android/Maven application project. This is to be compatible with non-Maven developers' library projects, which is important to grow the Android developer community.
Use other non-Maven developers' libraries
It also means we can take any external Android library project zip file (from non-Maven developers) and do mvn install:install-file ... on it and simply start using it as a dependency.
Share your .apklib with non-Maven developers
To share your .apklib file with a non-Maven developer, they will probably feel more comfortable if you rename it to .zip. They can then simply unpack it in a directory and use it from there.
If it is a mavenized Android library project, simply run mvn clean install
to create the apklib and get it installed in your local Maven repository.
If it is a regular Android library project, simply zip the project folder (better to strip all unnecessary IDE generated files), rename my-lib.zip to my-lib.apklib, then run mvn install:install-file
to install my-lib.apklib into your local Maven repository.
Note that in your multi-module maven project, if the child module lib2 is not mavenized, you may get some configuration error when running maven build, it is called multi-module Maven project after all so does not make much sense to include a non-mavenized child project. If the lib2 is not mavenized yet, either mavenize it as part of the mutli-module maven project or remove it from mutli-module maven project and maintain it separately (from both command-line build via apklib and IDE development via the source).
Also note that project building in Maven and Eclipse are totally different story, if you are creating/importing the project into IDE, you need the library project source anyway. If lib2 is mavenized as part of the multi-module project, it will be imported and setup automatically when import the parent project, otherwise, you may need import and setup the standalone project manually. Check out this answer and this answer for more details.