Not saying that's right. Go ahead and track the guy down using his IP address, haul him into court and teach him a lesson. But a "credible" death threat it is not.
As you say, you would have to be profoundly stupid to threaten FBI agents in a serious way.
However, we must remember that it is also NOT against the law to openly wish that all of these FBI agents die in a grease fire wrapped with Garland and Wray..