Seems like one of two things was going on here:
1) This was real, and the whole point was to recruit this guy to do computer work at the FBI, and the whole "we found you from post you made" was simply an "in" to get the two parties talking. Basic employment head hunting.
2) These were NOT FBI agents, and someone may have reported him to some kind of dark leftist/cabal group who wants to try and false flag him. Imagine if you had a friend who was a leftist and knew you used 8chan. Now imagine that they reported you to some antifa like group. Then that group sends out goons to try and get info on you and then use you as a patsy in some kind of false flag…
Honestly, who knows.