I've looked at the video four times now, and I can honestly say I can't see the problem
If you look at the white van in front of the black car, and then the humongous lorry+trailer, they all appear to be travelling at the same sort of speeds whilst in the l/h lane.
maybe a little timid coming off the slip road onto the m/way, but I can't say it is ridiculously/unsafely slow.
Strongly suspect the driver of the black car, rather than keeping pace with the camper van and joining the motorway in front of the black lorry, saw in its mirrors the black lorry with trailer and slowed down. There by causing the black lorry to move to lane two, and most likely causing the traffic behind the black lorry to brake or slow heavily to 56mph.
If the driver of the black car had been competent, both it and the camera car would have been up the road and all would be well. No lane changing lorry, no braking by oncoming cars to 56mph, etc.
If the lorry had not moved to lane two, I'm fairly sure we would have seen the black car stationary at the end of the slip road waiting for a gap in the traffic. Then joining the motorway from zero mph.
At least, that's what I saw anyway.