I see a lot of discussion there, but I'm not sure that a conclusion was actually reached. 
To me, a "core" is mostly the chip-specific code that enables a whole class of devices, while a "platform" is a specific product or group of products. So the "digistump" board package is a "platform", while the MCUDude and SpenceKonde (MegaCore, ATtinycore, etc) are "cores." (in particular, a "core" development effort focuses on the "cores" directory tree in the source, while a "platform" is more about the boards.txt and "variants" tree...