Is it time to start calling out bad cyclists?

Driving along fairly twisty forest road on Sunday morning. Group of riders on Sunday club run (around 30) come around the corner using both sides of road. I had to pretty much come to a complete stop as they clearly couldn't get themselves back in a bunch quickly enough to get on the correct side of the road.
I gave a little toot on horn to make a point and the riders towards the back decided to flick the Vs at me as though I had done something wrong. Am I wrong in expecting them to have actually made a sorry/thanks gesture for not actually running half the group over!

Did the road have a white line in the middle?