... Ok, that is legitimately impressive, from a technical standpoint.
Lua is a high level, not exactly very 'fast', very performant language. It is designed to be very, very human readable, and coding noob friendly.
Getting a 3D physics engine to work ... in lua... is not something I would have thought possible.
Usually you need to use a much lower level language to ... actually do that.
EDIT:
A few other commenters have now pointed out that this is actually using LuaJIT... which passes Lua code to a C compiler, quickly translates and then compiles in C, and then runs in C.
So, that makes much more sense, its functionally running in C, a lower level, compiled code language.
Still impressive nonetheless!