I'm not quite following here -- how does stopping executive-branch employees from stepping far outside of their technical expertise into the world of statutory interpretation and constitutional law stop them from continuing to conduct their duties prescribed by law, as explained by the judiciary?
They absolutely can apply the law to specific factual scenarios. They just can't necessarily apply new legal theories to new factual scenarios without getting challenged. The words "apply the law" generally do not mean "invent new legal theories."
Technically they still can do that but they can now be overridden by any judge for any reason. This is an obscene power-grab by our most corrupt and least accountable branch of government.