You did right to carry on as you were on the other side of the carriageway, though you would do good to contact the police as a witness.
I’ve had a similar experience where a car on the opposite carriageway had left the motorway, barrel rolled along the verge and hit a sign on the embankment, didn’t see the impact but saw cars scattering. Immediately burst into flames, everyone slowed down on my side and it turned into chaos, cars barrelling down on me trying to stop whilst people are running across 6 lanes of traffic and vaulting the central barrier with piddly little fire extinguishers.
The lorry drivers had it pretty sorted though, as they stopped they had created a line from the hard shoulder out to the second lane to shield the scene from traffic. I drove out of the chaos, pulled over for a few seconds but then decided that there was nothing I could do so carried on.