An importer accepts product that is developed and made by a non-domestic manufacturer without any input from the importer. The importer may place an order for such items to be produced, but the product, design and distribution of the product is totally in the hands of a manufacturer.
A manufacturer develops its own products, regardless of where it is made. In the U.S., a manufacturer may either produce products domestically, contract to have the work done elsewhere, or produce products outside the country in nondomestic plants. If it is produced elsewhere, the product is made exclusively for the manufacturer that commissioned it.
The line has blurred somewhat in that, at times, some model railroad manufacturers seem to be bringing in "what's available" instead of playing a direct role in a model's production. And, in the case of China, almost all of the work, including research and development, is now being done overseas. There is also some question about whether the tooling can eventually be used for another company, since China wants to retain all tooling, even when it was made for a specific American company.
But, for the most part, O gauge manufacturers are still "manufacturers" at this point, technically speaking. They specify what product should be made, play a role in the development process (especially in electronic circuitry that is proprietary, such as Legacy and DCS command systems) and hold exclusive rights to those models.
In our hobby, an example of an importer is Walthers -- not for its Proto (formerly Life-Like) line of trains but for everything else in its product catalogs. It has no hand in the design of most of the products, such as those made by Kibri, Vollmer, Heljan and Tomy (Tomytec and Tomix).
So it is correct to say there are both importers and American manufacturers in our hobby.