As you said, the road was wet and greasy. Probably some diesel there as well.

From the footage the accident was nothing to do with you, it's just one of those risks of being on two wheels. Wanting to go and assist the rider is a different matter. In the UK I don't think there is any legal requirement to do so. As a biker I would have stopped to help. Keep that camera footage just in case any nonsense comes later such as saying your braked hard or something.

No fault of yours at all. A mix of rider error and what looks like a spill of diesel on the road.

To me it looks like his attention is elsewhere, so he misses that you started to decelerate. When he looks back to where he's going he sees he is suddenly much closer to you, and grabs a handful of brake and locks the front wheel, not helped by there seeming to be something slippery right where he's riding.