It's such an extraordinary claim that, without evidence, it's reasonable enough to dismiss. The causes of Chernobyl have been pretty exhaustively thrashed out, and it all seems pretty well explained without any hacking intrusion.

If every claim like this has to be met with a shrug and "I guess we'll never know the truth" then it becomes very hard to ever know anything, because there's always some nutter willing to claim something that's totally at odds with all the available evidence.