I think the French saying is a response to rising Fascism. If Fascism is taking power in your society and you do not directly and explicitly oppose it, then you are at the very least going to be tacitly supporting it. By definition, centrism cannot fight Fascism, because it will always “both sides” the problem, which ultimately only benefits the Fascists. This is what we are seeing now in the US with the Democrats. The only realistic opposition to Fascism in this scenario is Leftism.
I think the French saying is a response to rising Fascism. If Fascism is taking power in your society and you do not directly and explicitly oppose it, then you are at the very least going to be tacitly supporting it. By definition, centrism cannot fight Fascism, because it will always “both sides” the problem, which ultimately only benefits the Fascists. This is what we are seeing now in the US with the Democrats. The only realistic opposition to Fascism in this scenario is Leftism.