My question also.
The video report ends with a statement that the comet’s trajectory has been tracked and is projected to take it out of the solar system completely.
Maybe that’s what leads them to the conclusion that it hasn’t been here before.
I dug a little deeper. It’s in a hyperbolic orbit or It’s going fast enough not to be stuck in orbit around the sun. The second reason is that gases like oxygen are evaporating from the surface. Since its going nearly to Mars orbit that stuff would be long gone if it had came by before.