With LTO when you "compile" a source file it really only processes it into an intermediate language (IL) format and stores that in the object (.o) files. When you invoke the linker on those object files it runs the compiler again with all the IL from the inputs and generates a temporary object file with the actual executable object code, which is then linked into the final executable binary.

I would guess that mold doesn't have the necessary support for the second step, running the compiler again on the IL during linking.