He was referring to the default aliasing rules. By default Fortran assumes that dummy arguments to subprograms don’t alias. An example of this in action can be seen here: C Is Not a Low-level Language - #25 by ivanpribec
One may run into aliasing inhibiting optimization when using pointers in Fortran. See this topic for some guidelines what can be done in that case: Fortran pointers, and aliasing