Yes, primarily. But the real answer is because of Lua from Garry's Mod, and the design of the engine is heavily inspired by Source and QuakeWorld. Even today, I still believe Quake is one of the most elegantly designed game codebases in existence.

Planimeter was formerly called Team Sandbox, and we were a group of developers working on a game called Half-Life 2: Sandbox, which was a FOSS alternative to Garry's Mod that stayed Half-Life themed.

The team is comprised of mostly different volunteers now.