The answer is simple. Watchtower uses secular information to establish the starting point for their own calculations, after which they condemn secular information.
“Find the hypocrisy and ye shall be free”
Its called “poisoning the well” after you have drank your fill.
If you throw “secular dates” out the window from the outset, then 607 disappears, simply because you would have no starting point.
The Nabonidus Chronicle establishes the starting point, and that is “secular history”, which Watchtower first uses, then condemns.
https://en.m.wikipedia.org/wiki/Nabonidus_Chronicle
... by the way, the unScholar dude knows this, but he is not interested in the truth.