Another clue showing Simonides wrote Sinaiticus is Simonides said: "But 1 may just note that my uncle Benedict corrected the MS. in many places, and as it was intended to be re-copied, he marked many letters which he proposed to have illuminated."
https://archive.org/details/sim_journal-of-sacred-literature_1862-10_2_3/page/250/mode/1up

In Sinaiticus' Folio 262 (also called Quire 82 Folio 2r), Sinaiticus' page of Romans 3:5-4:12, the red arrows point to the long areas (on the page's left and right side) that Simonides' uncle Benedict would illuminate, and to notes in the page's middle margin
https://codexsinaiticus.org/en/manuscript.aspx?__VIEWSTATEGENERATOR=01FB804F&book=37&chapter=4&lid=en&side=r&zoomSlider=0

Also there's lot of long Sinaiticus notes in the bottom of other pages Link & Link