No there is a distinct difference between not being able to stop the sale of bump stocks and being able to stop the sale of bump stocks. As a matter of fact it is probably as clear of a distinction as you will ever find in life.
Apparently,they decided they had no jurisdiction to regulate it because of how they chose to characterize the item. I haven't researched the details but your interpretation seems reasonable. We are just using language a little more loosely. They could have interpreted the device as a modification that allowed full auto fire and then they would have had jurisdiction, at least as I understand it.
The ATF made the WRONG judgment. A gun part that changes a semi auto to almost a fully automatic rifle is their jurisdiction. That is IS the point. BTW, this was the same ATF that thought it was in their jurisdiction to allow illegal arm sales to drug cartels.