The issue is largely rhetorical; BG2 could perhaps more rightfully have been called a DLC (this was before the internet made DLCs prevalent) as opposed to a full game/sequel.
in what world? It doesn't require BG1 to be played, it improves the engine and systems, provides far more content then BG1, has it's own self contained story (even if it direct sequel it doesn't require BG1 to be played and be enjoyed. As a matter of fact I always recommed to start with BG2) - overall it's more of and better then BG1.
