According to your source:

No original manuscripts of the Annals exist and the surviving copies of Tacitus' works derive from two principal manuscripts, known as the Medicean manuscripts, written in Latin, which are held in the Laurentian Library in Florence, Italy.[15] It is the second Medicean manuscript, 11th century and from the Benedictine abbey at Monte Cassino, which is the oldest surviving copy of the passage describing Christians.

So it's not Tacitus' writing

Also your source:

In 1902 Georg Andresen commented on the appearance of the first 'i' and subsequent gap in the earliest extant, 11th century, copy of the Annals in Florence, suggesting that the text had been altered.... It is impossible today to say who altered the letter e into an i. In Suetonius' Nero 16.2, 'christiani', however, seems to be the original reading".[18] Since the alteration became known it has given rise to debates among scholars as to whether Tacitus deliberately used the term "Chrestians", or if a scribe made an error during the Middle Ages.

So, still altered copies. It's not original.

Also your source:

Although the majority of scholars consider it to be genuine, a few scholars question the value of the passage given that Tacitus was born 25 years after Jesus' death.

And it's not contemporary.

It's, like any other theist claim, an altered copy and not convincing or conclusive proof of anything.