AIUI (IANAL etc), there is an amazing problem at play here. As I see it, domestic terrorism is perfectly well defined but the wording of the penalties seems to be rather explicitly lacking (see "outside the United States" wording) provision for prosecuting domestic terrorism as terrorism.

Roof intended his slaughter to intimidate and coerce (the reintroduction of segregation policies). It was clearly domestic terrorism by the above wording.

Bombing medical clinics that provide reproductive care isn't typically prosecuted as terrorism either, even though they are dangerous acts where the intent is to intimidate and coerce (the abandonment of roe v wade). It likewise is clearly domestic terrorism.

I suspect that it's not that these things wouldn't hold up in court, but that the powers that be believe there is no gain by prosecuting it as domestic terrorism. I'm not sure if that's what Lynch wanted to say, but in context it is essentially what she said. I expect that making domestic terrorism a prosecutable definition (or whatever the terminology is) would involve admitting there is a domestic terrorism problem, and one on multiple fronts. What easier way to pretend there isn't a domestic terrorism problem than call domestic terrorism by another name.