There's a history between Tastic and Core, but it's a different one. Meshtastic doesn't scale that well in urban areas and it seems that some on the Mestastic team didn't see that problem as a priority/ignored the problem/are too stubborn. And then Meshcore is created with a different routing, works much better in practice, proving that the mesh could be much better. In countries like the UK it seems to have replaced Meshtastic in most places.
There's no mention of Meshtastic.
So the split is just a "the development team has nothing to do with the .co.uk site, his youtube channel and discord group", not a fork.