I think what that means is "this guy", an acquaintance of the person who is writing the comment, is like a real hardcore jackass mercenary who has seen and done some horrendous shit. The acquaintance is unrepentant, and fears no consequences, and freely shares accounts of his various crimes. He has been in and out of military work for decades so is presumably a known, trusted element and called in when ruthlessness is expected. This guy liked to tell stories to shock the sheltered sensibilities of the commenter and took some joy in emphasizing the brutality in his own stories.

But even this guy "definitely never told me anything this bad." Referring to the video. As in, the video is making more extreme claims than the commenter's acquaintance ever did.

My reading is that the commenter is not corroborating. Actually it's more like they are having a bit of doubt regarding the veracity because it seems outside the boundaries of even what they have learned about this kind of business from the guy they knew.