Bp Williamson was wrong when he said that... And it is not necessary to be French to think that. Every traditional catholic can understand it easily. Saint Paul said in gal. I, 7-9 " [7] Which is not another, only there are some that trouble you, and would pervert the gospel of Christ. [8] But though we, or an angel from heaven, preach a gospel to you besides that which we have preached to you, let him be anathema. [9] As we said before, so now I say again: If any one preach to you a gospel, besides that which you have received, let him be anathema."
The modernists do not preach the same faith. The actual pope would be declared anathema by saint Paul.... So we cannot go to a mass of someone who would be declared "anathema" by saint Paul. Saint Thomas explains that "Anathema" means : "cut, separated from". So saint Paul teaches us to be separated from those who have not the same faith.