By your own admittance, though, the Guardian got their information from Simonides, which means it does not qualify as corroboration.
Corroboration would be eyewitness testimony (it would be quite difficult to conceal efforts at forging a copy of the entire New Testament in a busy monastery), or other information. Since Tischendorf technically never met with Simonides, he doesn't really corroborate it, either.
There is physical evidence in the form of two lithographed letters that Simonides sent to his friend describing the manuscript he was writing.
Those letters are in the British library. The foreman at the lithograph company attested to them.
Your corroboration is the lack of evidence. Is it not equally possible that the codex was simply lost to memory before its rediscovery?
Yes it is possible, but there is quite a bit of circumstantial evidence.
Letter dated April 19, 1853 from John Hort to Reverend John Ellerton.
Excerpt-
"He(Westcott) and I(Hort) are going to edit a Greek text of the NT some two or three years hence, if possible. Larchmann and Tischendorf will supply us with rich materials(Codex Sinaiticus), but not nearly enough,--.
Timeline of Tischendorf's finds.
1844 According to Tischendorf he was allowed to purchase 43 sheets of the Codex Sinaiticus from Mt Sinai, which he named Codex Frederico Augustanus. (1 Chronicles chapter 11-19:17, Ezra 9:8-Esther 10, Jeremiah11-Lamentations 2:20)
1853 Tischendorf is back at Mt. Sinai looking for rich material for Hort. Unable to find more of the Sinaiticus.
1859 Tischendorf miracusly finds every single leaf of the Codex Sinaiticus New Testament in whole condition.(Hort was not concerned with the Old Testament.) The Old Testament portion was fragmented with much of it missing. (Remember this is the same Codex Sinaiticus that the greatest forger of the nineteenth Century claimed to have written.
The strongest proponent for the codex was Henry Bradshaw. Henry Bradshaw was Hort's best friend. While at Cambridge they worked together to put down a revolt against the Vatican.
Then there was the fact that only Simonides had seen a partial copy of the Shepherd of Hermas in Greek, and the Codex Sinaiticus ended in the place in Hermas as Simonides had recorded.
Best regards, Terry