This does not really "guarantee" anything. You can still have as much UB (incl leaks) as you want, as long as they are not evaluated at compile-time. i.e. it's at best equivalent to running valgrind.
The point is that the code is only checked for UB with the arguments it is given at compile time. There is no guarantee that it can't invoke UB with other arguments it might receive at runtime.
For example, here is a modification of the original program that does invoke UB, but compiles just fine: