Except that the Supreme Court didn't find the ATF's regulation "unconstitutional." Rather, the Court ruled that the regulation was contrary to the plain language of the pertinent provisions of the National Firearms Act of 1934. Those are two different things entirely. Congress is not without authority to change the language of the National Firearms Act to prohibit bump stocks.
In his separate opinion, Justice Alito said exactly that:
"There is a simple remedy for the disparate treatment of bump stocks and machineguns. Congress can amend the law—and perhaps would have done so already if ATF had stuck with its earlier interpretation. Now that the situation is clear, Congress can act."
Yes, an agency cannot act outside of the respective “enabling statute” which “canalizes” what the agency can or cannot do