Portals in Corona aren't really a light sources, they just tell renderer which direction should be sampled more.
Simplified explanation:
It will help in situation where mesh light steal sampling from object lights. In older build, if you made CoronaLight sphere and Sphere primitive with 64 polygons and LightMTL on it, then Sphere was considered to be 64 lights. So then, when lighting was sampled, there was considered to be 65 lights in scene, and CoronaLight sphere had equal importance as the remaining 64 of those polygons.
After this fix, polygons are not separate lights anymore, but grouped by the source objects. So one teapot is just one light, three teapots are three lights, and if you attach these three teapots together, they will be considered one light.