Tell me more about the comet sightings at that time.
This is usually read as a reference to the invasion of the Saxons into Souvrn' England.
What we now know is there was no large invasion, no burning of the country, and people didn't just die with no signs of wounds ~ as indicated in Gildas.
You could have a smaller comet behave like a meteor bolid that would swoop in a great arc over a considerable part of the planet. Depending on how close it was, it could start fires as it passed. People on the ground would have seen it coming for months possibly. The noise would have been absolutely incredible if it managed to penetrate the atmosphere.
As it passed beyond the atmosphere in the far West it would have seemed to have first disappeared into the sea.
So, yes, that's one eyewitness. There are others. It's the basis for the end of the Annals of the Kings of Britain ~ the END OF THE WORLD ~ and then all the characters become survivors.
Myrrden had to replant the grapes in Brittany, and on the upper reaches of the Rhone Valley. Probably got frozen out. And who's Myrrden ~ isn't he from Europe? Why did these guys move from Britain to Brittany and France ~ where were the people ~ didn't anybody think to stop them? Tree rings from that time even show several years of stunted growth.
The caravans crossing the Great Sind in Baluchistan moved on, a few dogs barked! No one noticed that Europe and China were missing!