That's what I'm thinking. They've subtly incorporated a few things from FC2 into FC4 like controlling how long you sleep, the tracker that Longinus gives you. When he said he was a warlord I immediately flashed back to my adventures in Africa. That game was all about blood/conflict diamonds and warring factions. I think it's very possible that our mercenary from FC2 was the one who squeezed that trigger, if not him then he had something to do with it. I always love to see small things like this that have connections to previous games that not everyone may catch.
Edit: He could be Doctor Leonid Gakumba. Just a thought. You did have to assassinate him, he could have survived.