That is a total mischaracterization of what it means for a language or DSL to be "powerful." When you are writing not-X to assist X in doing Foo, that doesn't mean X is powerful-enough to let you do Foo; that means that X isn't powerful-enough to let you do Foo without breaking out of X.

To put it another way: the fact that you can greenspun Lisp in C doesn't mean C has all the power of Lisp. It means that Lisp has all the power of Lisp. Every even-mildly-useful language is Turing-complete; so, in order to be able to give a ranking to languages at all, we have to restrict our discussion of a language's merits to its merits if you aren't just using it as a glorified Turing machine for implementing a different language.

Without writing custom PHP, Drupal is less powerful than PHP. That's perfectly okay.