This is referred to as shifting the Overton window. If voices from the extreme are not heard, the Overton window moves away from their position, so protests help their cause even if only a minority completely agree with them.
I don't think Overton implied any causality between the phases of the window, just that distinct phases exist and that forces act on the window to cause it to shrink, expand, and shift.
Interestingly, considering my post above, I think that view of the Overton window misrepresents what Overton thought.
It was first and foremost a decription based on observation, but I think it conveyed that he felt that the Overton window was moved by vorces in the middle, and forces at the extremes moved it in the other direction.
There is a distinction to be made of how strongly the opinion is held compared to how much the opinion differs to the median view.
I think there is a correlation at the extremes, but I think the Just Stop Oil approach is deliberately splitting those two, performing extreme actions to promote a view that is not terribly extreme.
This is consistent with the view of the Overton window that the extremes repel and the moderate attracts.
Compare that to a vegan who doesn't campaign, but silently judges every meat eater as a murderer. If their views were more public, it may drive people the other way.
https://en.wikipedia.org/wiki/Overton_window