I'm wondering what source was consulted to arrive at that answer?
Main articles: Antichrist, The Beast (Revelation), and Number of the beast
The Sibylline Oracles, Book 5 and 8, written in the 2nd century, speak of Nero returning and bringing destruction. Within Christian communities, these writings, along with others, fueled the belief that Nero would return as the Antichrist. In 310, Lactantius wrote that Nero "suddenly disappeared, and even the burial place of that noxious wild beast was nowhere to be seen. This has led some persons of extravagant imagination to suppose that, having been conveyed to a distant region, he is still reserved alive; and to him they apply the Sibylline verses." Lactantius maintains that it is not right to believe this.
In 422, Augustine of Hippo wrote about 2 Thessalonians 2:1–11, where he believed that Paul mentioned the coming of the Antichrist. Although he rejects the theory, Augustine mentions that many Christians believed Nero was the Antichrist or would return as the Antichrist. He wrote that, "in saying, 'For the mystery of iniquity doth already work,' he alluded to Nero, whose deeds already seemed to be as the deeds of Antichrist."
Some modern biblical scholars such as Delbert Hillers (Johns Hopkins University) of the American Schools of Oriental Research and the editors of the Oxford Study Bible and Harper Collins Study Bible, contend that the number 666 in the Book of Revelation is a code for Nero, a view that is also supported in Roman Catholic Biblical commentaries. The statement concerns Revelation 17:1-18, "the longest explanatory passage in Revelation", which predicts the destruction of Rome by work of an eight emperor who was also one of the seven kings of the most extended and powerful empire ever known in the human history: according to this lecture, Babylon the Great is identified with Rome which has poured the blood of saints and martyrs (verse 6) and subsequently become the seat of the Vatican State, reigning over all the kings existing on Earth."https://en.wikipedia.org/wiki/Nero#Antichrist
The NEB & REB, RSV & NRSV Oxford Study Bibles give Nero as the most likely person referred to as 666, as does the Harper Collins Study Bible mentioned in the Wikipedia article. In addition, The New Interpreters' Study Bible, NRSV (2003) gives Nero as the probable identity. I've checked these personally and the Wikipedia is correct in making those claims. The Roman Catholic Study Bible, NAB Second Edition also gives Nero as the likely person meant. BUT
, at least one other commentator who is NOT a liberal understands 666 to be Nero and I refer to David S. Clark in his postmillennial, Preterist commentary on Revelation, "The Message from Patmos" and this commentary is available online. Rev. 13:18 is discussed on page 91: https://archive.org/details/messagefrompatmo00clar/page/90/mode/2up
For myself, this commentary by David Clark gives the most natural, clear reading of that difficult book.