I disagree. For an open core product the core is actually open source. You can fork it, change it, distribute it. It may have an OSI approved license. You can't do that with a source available product.

Furthermore, you can't even talk about the open core as a part of the closed source product, because the open core application is invariably a whole in and of itself. You could theoretically fork it, improve on it and it could have a life on its own as a 'fully' open source product. You can even make it incompatible with the closed version.