Parking is extremely difficult and can be quite expensive. There are a myriad of parking signs on the streets, which can be rather complicated to understand. Vehicles displaying placards can be ticketed and towed if parked improperly. Wheelchair riders can get to most sightseeing spots by wheeling, however depending on your route, you'll likely encounter steep hills. The underground trains, most buses, and plenty of curbcuts make getting around very easy.
Ramped taxis are also available.
Truth is, nobody knows who really said it. Regardless of its origins, do take the advice seriously. Unlike most places, summers in San Francisco are unpredictable and often cool and foggy. The best weather months are April, May, and September through November. The rainy season is December through March.