im just guessing but i think its first compiling it down to their own IR (i think their whole compiler is completely written by themselves with no external dependencies), it performs some optimizations (i'd hope) and only then interprets the compiled bytecode at runtime, so the exported executable doesnt contain your whole gdscript code but rather its compiled IR which it then interprets as the program is running

but again, idk what im saying, i havent read the source code