Up close, it was clearly real flame. I did see a bit of posterization in the long shot, but I took that to be just an artifact of the video's resolution, though it could've been a function of the compositing.
Either way, "CGI" is the wrong term for that. That stands for computer-generated
imagery, i.e. an image created entirely within a computer with no live photographed elements. I've seen CGI fire and it looks totally fake. At most, this was a live pyro element that was digitally composited into the scene in the few long shots they did. That's not computer-generated, just computer-manipulated.