I'm not talking about the protest, I'm talking about the driver that chose to drive his car through a crowd for a political end. Politically motivated slayings directed at civilian populations to my recollection have usually been pretty easy to call "terrorism," whether it was the Unibomber, the Weather Underground, the IRA, or Timothy McVeigh. Motivated by high-minded, low-minded, or trivial goals, they all had one thing in common; kill or injure people to send a message to their political rivals, be it governments or other citizens.
There's a really easy test here: If this had been a Muslim driving through a crowd of people would we be parsing the phrase "terrorism" so finely? There is no slippery slope here, because I'm not calling the act of protesting an act of terrorism, I'm very narrowly focused on the politically motivated slaying, which appears to be directed at preventing taking down a statue of Robert E. Lee -- a fairly trivial political end, but a political end nonetheless.