Do SiO2-infused shampoos and spray coatings actually add protection?

They can add a thin, short-lived layer that improves water behavior, slickness and ease of cleaning. That layer is far thinner and less durable than a properly applied ceramic coating, and its durability depends heavily on the formulation. These products are best used as maintenance toppers or for cars without a coating, not as a replacement for surface preparation and a full coating.