Second witness to Simonides writing Sinaiticus: Frederic G Kenyon, a British paleographer who was the British Library's Director and Principal Librarian from 1909-1931 (Link) mentioned that Simonides made numerous forgeries: "Since the year 1856 an ingenious Greek, named Constantine Simonides, had been creating a considerable sensation by producing quantities of Greek manuscripts professing to be of fabulous antiquity, such as a Homer in an almost prehistoric style of writing, a lost Egyptian historian, a copy of St. Matthew's Gospel on papyrus, written fifteen years after the Ascension (!), and other portions of the New Testament dating from the first century. These productions enjoyed a short period of notoriety, and were then exposed as forgeries.
(Our Bible and the ancient manuscripts, Page 124)
https://archive.org/details/MN41613ucmf_0/page/n160/mode/1up

Even though Kenyon later claimed "it was shown that Simonides could not have completed the task in the time which he professed to have taken;" (Page 160), this is overruled by Simonides himself stating both Simonides admitting to writing 1,205 pages of Sinaiticus in 8 months and by stating Constantine Von Tischendorf's admission of Simonides' education and worldwide travels: "Dr. Tischendorf, even, says as follows : - Simonides received an excellent education, and, when a young man, spent a considerable time in the Greek monasteries at Mount Athos, occupying himself almost exclusively in the study of ancient MSS., by means of which, especially," (Page 229)  he greatly enriched his knowledge of the Greek and Egyptian antiquities. At the same time he employed himself very much in drawing and lithography, in which he became very skillful ; and this skill was turned to account afterwards, when he copied the ancient MSS. Moreover, by vigorous study and many voyages in Asia and Africa, Simonides arrived at the climax of superiority io philology, particularly in the knowledge of the wonderful art of ancient MSS.  ... I admit that, according to your judgment it may be so, but according to mine — I, who am called Daedalus by some of the journals, it is quite possible, and by no means laborious ; ...  I know that I wrote 1,205 pages in eight months,"(Letter of C. Simonides, The Journal of Sacred Literature and Biblical Record, April 1863
Page 229 Link and Page 230 Link)