It is a restatement of the antithesis to a very common logical fallacy; the argumentum ad populum or “appeal to the masses”.
The shorthand of the fallacy is, “a hundred million people can’t all be wrong”. It’s an appeal to an arbitrarily large number of people who, it is asserted, favor or support an idea, platform or person that one’s debate opponent is contesting. The implication is that these people are all intelligent, dispassionate human beings who have applied logic and reasoning to the question and all come to the same conclusion; therefore, you should too.
The reality, of course, is that 100 million people can indeed all be wrong. Every last one of them can be making an error in fact or reasoning, based on emotion, mistake of fact (whether error or omission) or “worldview bias” that colors the calculus of their logic, that leads them to an incorrect conclusion regarding the point at issue. It’s not even all that unlikely, depending on what or whom is at issue. They don’t have to be making the same mistake. They do not, in fact, have to be thinking logically or critically in the first place in order to assert the truth of a statement or the correctness of a concept or a way of behaving. To assert that you should do likewise flies in the face of the very system of logic being used to make the argument.