The perp was a diagnosed schizophrenic and the Va police have only charged him with 2nd degree... which means they don’t have probable cause at this point in their investigation to say it was terrorism.
There were also news reports quoting anonymous march organizers who implied that the march route was kept secret and that if it had not been for the death car looking like a cop car, it would not have been given access to the street. So either the driver had accomplices scouting for him, or the driver found himself on the street rather by accident.
There were two other cars on that street that were blocked by alt-left protesters after the main event. If you watch the drone video it looks like the protesters were disassembling and going their own way until the first of the two cars, a minivan hit, one or two of the protestors and was stopped. That stopped the white car immediately following. Protesters surrounded both cars blocking the view of the oncoming Dodge.