Checking the Github repo, it says 92.3% C[1], 2.6% Lua[2], why does the author claim it's a "text editor written in Lua" instead?
Excluding those, it's 1369 lines of C (.c or .h), and 5389 lines of Lua (.lua), and 112 lines of other files (license, readme, build scripts...); or 78.4% Lua, 19.9% C, and 1.6% other. I didn't count fonts or images (.ttf, .ico, or .inl).
Which is written in C.
Most of the implementation is oddly in the "data" folder.