The third clause explicitly states "Nothing in this Article shall preclude: (a) the inclusion or implementation of terms and conditions related to the provision of source code in commercially negotiated contracts;"
That is to say, the article is explicitly not prohibiting countries from enforcing copyleft licenses.
Most copyleft licenses are not, in the manner normally entered into, “commercially negotiated contracts”, and many are gratuitous licenses, not contracts at all.
That is to say, the article is explicitly not prohibiting countries from enforcing copyleft licenses.