ValidateMulticastCommand.java is not located in admin-cli.jar, it is in gms-adapter.jar.
Here is listing:
% jar tvf gms-adapter.jar
5575 Tue Nov 22 15:25:26 EST 2011 org/glassfish/gms/admin/validate-multicast.1
2427 Mon Oct 29 08:15:02 EDT 2012 org/glassfish/gms/admin/ValidateMulticastCommand.class
The source code lives in all/main/nucleus/cluster/gms-adapter/src/main/java/org/glassfish/gms/admin/ValidateMulticastCommand.java
Given that this CLI command is a wrapper around code from shoal-gms-impl.jar that implement validate multicast,
I believe this is a reasonable location for the code. If no one objects, I recommend closing this issue as not a bug.