I defer to any classicist here or a classical philologist but Tacitus comments in his ‘Historia’ never seem to be questioned as they are contemptuously dismissive of Christians and rate Jesus as a subversive hedge preacher. He makes the comments in discussing Nero’s search for a plausible scapegoat for the fire that destroyed a good part of Rome during his reign. Since Tacitus is considered a hostile witness and he gets several major items at variance to the orthodox Christian teaching in his comments on Christians and Jesus his writings are not considered a possible later interpolation.
Good point. A negative witness ought to hold more weight than a positive one, at least in the question of whether or not the subject of the negativity existed. IE why would a known credible historical source rant and rave against someone who they knew didn’t exist?