I'm surprised it's taken so long for them to figure it out, but thanks to the NRA it is so easy to get lots of high powered firearms, with no tracking or registering, it was really just a matter of time before lone wolf type terrorists started becoming more common in the US.
So, really, the NRA is, in fact, an enabler of terrorists. Note: this has actually been true for some time as lots of not-brown gun nuts and gun nut groups do employ terrorist techniques in the literal sense of instilling terror in others, particularly any opposition (think open-carry activists). Really, the NRA is guilty of this. So I suppose the [modern] NRA is both an enabler of terrorism and itself a terrorist organization.