That depends what the conclusion is supposed to be. If it is just that philosophers X, Y and Z are wrong, then you are right - he can simply bring forward arguments a, b and c to show this.

However, his claim is stronger than that. He is claiming that these philosophers (or at least many of them) are not in the truth business. His philosophical arguments may show that the other philosophers are wrong, but it won't follow that they are not sincere in trying to find answers and solve problems. For that he needs something like: they can't really be trying to find the truth, otherwise they would agree with me (at least on these "simple" matters).