How a pinnace could be both ship's boat and sailing craft

Was a pinnace a type of ship's boat?

Yes. It was often among a warship's largest boats and could also sail meaningful distances independently.

A pinnace commonly served as one of the largest boats carried aboard a ship. Before motor cutters, it could be a substantial sailing craft in its own right. Its rig and endurance allowed it to travel farther than a small pulling boat. That dual identity helps explain why period terminology can blur the boundary between a ship's boat and an independent small vessel.