Some mythicists argue that the reference in Tacitus was not actually written by him—they claim the same thing for Pliny and Suetonius, where the references are less important—but were inserted into his writings (interpolated) by Christians who copied them, producing the manuscripts of Tacitus we have today. (We have no originals, only later copies.) I don't know of any trained classicists or scholars of ancient Rome who think this, and it seems highly unlikely. The mythicists certainly have a reason for arguing this: they do not want to think there are any references to Jesus in our early sources outside the New Testament, and so when they find any such reference, they claim the reference was not original but it was intended by Christians. But surely the best way to deal with evidence is not simply to dismiss it when it happens to be inconvenient. Tacitus evidently did know some things about Jesus.