It is listed in their whitepaper v1.6 on page 39, I also found v1.3 on buffered googlesearch https://webcache.googleusercontent.com/search?q=cache:C4QjokDPJjgJ:https://www.sophiatx.com/_data/_custom/SophiaTX_Whitepaper_v1.3.pdf+&cd=1&hl=no&ct=clnk&gl=no&client=firefox-b-ab

So by what I can gather this has been in their whitepaper for a while.