Hard to tell due to the very wide angle lens, the chain looks too loose for a roadie, but on an XC it needs to be slacker to cope with the longer travel suspension.
In practice you wouldn't be running off road at a constant speed and your wheel would be moving around a lot more so I'm not sure what value the video is, most chains on all sorts of drives will exhibit the occaisional whip like the one time in your video, normally worse if there's too much slack.
Sorry but you need to fix a cheap bullet cam to a footrest maybe and then take it across a field to see if this is a problem or not and even then ask yourself why there's a rubbing strip under the swingarm if the chain wasn't going to contact it every now and again.
Other thinks to look at are the rear mudgaurd and number plate, mine clank when I jump off speed humps as does the hugger, but presumably you don't have one of these fitted? having said that chain slap is quite a distinctive noise and not normally mistaken for a mechanical clunk