You are correct, it is not possible to import releases via command line. Maybe start a thread in the ideas and enhancments thread on this forum.
I understand that the reason for no command line for releases is the complexity of importing releases, if you think about all the options presented to you in the import release GUI related to overwriting existing processes/objects, publishing processes, importing credentials, setting environment variables, etc,etc,etc. There is good reason why this is a manual task.