It is terrorism, indisputably. Arguing other point is relatively inconsequential. But if we were to argue the point, to argue that spray-paint isn't "violence" isn't going to have much credibility when it's perceived to be from the long standing "words are violence" and "silence is violence" crowd. A selective standard is no standard at all. How the spray paint differs is that it is property destruction in service of intimidation toward a political end.
Does that mean that Republicans engaged in terrorism when they threatened to hang journalists, shoot immigrants or Democrats, etc.? Seems like the courts might be overloaded if we redefine every threat as an act of terrorism.