This is a dumb argument. For starters, he calls the Christians "Chrestians" after their leader, "Christus".
For second, you have no idea what his source for his information was. He was also one the
Quindecimviri sacris faciundis who were responsible for overseeing foreign religions in Rome, so this may well be primary material of his own. We don't know.
For third, you're holding this to a modern standard of historicity. The ancients did not have this practice of sourcing.
And finally, Tacitus does not say
they say. He reports it as fact. If you call this fact into question, anything Tacitus writes in any of his histories that he either does not source or was not an eye witness to becomes "hearsay." This is a ridiculous anachronistic standard.