Hyperbolic.
Let's agree for the sake of discussion that by domestic terrorists they mean 75 million of Trump voters (which they likely don't). That doesn't nearly account for half of the country's population.
It is much likelier though that they refer to the fringe and militant groups that have recently manifested themselves; those that do not shy away from using violence for political purposes. Groups that
have managed to recruit thousands of members from the military and law enforcement agencies and are increasingly willing to undertake bold steps in support of demagogues like Trump.