how could the young Simonides do all that intense textual work in a short period?

Steven Avery

This question, and variants, are asked all the time.
And the answer is right in the history.

German book Lilia.jpg
[...] he [Benedict] recalled me to Athos. I sailed from the Pireaus in the month of November, 1839, and landed again at Athos for the fifth time. After a few days I undertook the task of transcribing the Codex, the text of which as I remarked before, had many years previously been prepared for another purpose" (Simonides, The Guardian 21. Jan 1863; published in The Journal of Sacred Literature and Biblical Record, 1863, Bd. 3, 228-229; vgl. daz.u auch Stewart 1859, 4 und Elliott 1982, 177).
Die getäuschte Wissenschaft: Ein Genie betrügt Europa – Konstantinos Simonides

Also in the:

Christian Remembrancer (1863)

Today we know more about that prep work.

David W. Daniels is doing research on the learned Benedict.

And we have one of the sources available, the Zosimas Moscow Bible of 1821.