Its true. Both creators have been asked about the situation. JV doesn't speak or read japanese so theres no way he could have read any of the manga unless someone read it to him. MY said he was aware of Invader Zim but was not inspired by any way it.
Keroro Gunsou manga DID start about 16 months before Zim. However Zim was in production for about this long anyway.
Either one could have ripped off the other, but each rip off is a very unlikely scenerio.