No, core is an excuse still, just in the opposite direction - "It's core, allow it." And the fluff attached is specific enough to be hostile to worldbuilding. On one hand we have specific fluff removed from monsters, where it does no harm, and attached to PC races, where "tieflings had an empire" is a restriction enough to require ignoring in countless worlds. It's bizarre.See, I find that as a good thing, since then "core" isn't an excuse. It means that the player and DM have to both discuss what they wish out of the game, including stuff like what races there is, how they may be refluffed, etc.