It seems that if he didn't explain his motives (i.e. art, not hacking), his actions may have easily been construed differently.
Almost always, if a federal agent is asking you questions while they are executing a warrant, they have built a prosecutable case against you – potentially absent any evidence they anticipate from the search. A United States attorney has certainly looked at the case and may have already obtained an indictment. The government has expended hundreds of hours investigating you. How much have you thought about defending your actions to the feds?
When a federal agent asks you questions they are looking for you to either divulge facts that they can use to strengthen their case against you, or they are looking for you to lie. You don't even need to intentionally lie: you might simply misremember facts that your investigator has reviewed intensively, you may omit something that your investigator already knows, etc. But if you do, you've made a false statement to the federal government which is a felony on its own.
Your lawyer can help you explain your motives without divulging things you shouldn't. More likely, your lawyer will tell you to say nothing because "explaining your motives" is another way to say "providing a confession".
You have nothing to lose from invoking your sixth. It's the only safe thing to do in this case.