Behavior of all involved parties is highly indicative that they at least believe the threats of violence to be real. Body guards, cancelled media appearances, unprecedented cancellation of a theatrical release, jumpy reactions in public, and more. Internet death threats are nothing special. They happen all day every day. Especially for celebrities. Hacks aren't an every day occurrence but we hear about them regularly.

That doesn't mean the threats are legitimate. Or that North Korea is involved. But the actions of folks involved do give serious weight to such an accusation.