Filip Pizło (https://www.filpizlo.com/), according to his website: Director of Programmability and Data Model at Roblox. Previously, I worked at Epic Games (Senior Director of Language Engineering) and Apple (Manager of the JavaScriptCore Team).

He seems to be the main developer of Fil-C: https://github.com/pizlonator/fil-c although there are other contributors.

Filip mentioned in the talk that Fil-C is his personal project.

I am asking, because I doubt a single person can achieve this. It requires at least a team of several people.