Edit: Actually this guy signed onto Unity in 2014 Unity was still privately held. It went public in September of 2020. Unity's founder, David Helgason is still on their board and served as some sort of Senior Executive whatever position at Unity. Dude still has a 4% share of their stock.
So anyway, probably this David Helgason character who hired him. And/or whoever else (including Riccitiello!?) who was on the board at the time.
The relationship between CEOs and boards of directors is so fucked up and incestuous...
In any case and in retrospect, Johnny boy was 100% hired to cue them for up for the eventual IPO and boy did he ever. It's amazing how fast companies go down hill when they become publicly traded.