The 'Radical Flank' is a strategy. It no more justifies right wing extremism, than it does me aggressively rejecting pineapple on pizza, to allow for a more moderate approach to pineapple on pizza. It allows for compromise.

They're all separate issues and it all boils down to what we believe as right and wrong. Extremism is also very much a spectrum. I'm sure it's needless to say that throwing soup at a painting to protect the planet is quite different to racism and anarchy.