You still need the borrow checker, and this compiler won't change the kinds of programs you can write. It only works on programs that pass the borrow checker, just like the regular compiler, the difference being that it doesn't check that the program is valid (i.e. passes the checker) but assumes that its validity has already been checked earlier by the regular compiler.

If you give an invalid program to the regular compiler it will be rejected, but if you give the same program to this compiler it might produce nonsense. So the developer of a program would feed it to the regular compiler for checking, but others could take the finished release of the program, assume that it's valid and feed it to this compiler instead, as I understand it.