Exactly. This was purely a technical decision. The ATF can only interpret the statutory law that Congress writes. They cannot rewrite it or invent new laws. A bump stock does not meet the statutory definition of a "machine gun".The upside of the decision is that the administrative entity has been properly limited; the ATF does not have the authority to make such declarations.
The downside is that some day, the Congress may take action; that would be the Constitutional approach. ...
Put differently, Congress can make bump stocks illegal by passing legislation. But Congress never did that, and administrative agencies can't make up new laws. PS: If Congress did pass new legislation for this, it could be challenged on constitutional grounds, and then the SCOTUS would have to decide the case again, this time on the merits.
This decision is being misinterpreted or misrepresented on most of the mainstream news channels I have read or watched.