From the announcement page: "Baldur's Gate: Siege of Dragonspear adds a new chapter to the Bhaalspawn saga. The events occurring between Baldur's Gate and Baldur's Gate II are at last revealed in this 25-hour expansion pack for Baldur's Gate: Enhanced Edition."
So yes, as confirmed by Beamdog earlier this year it is not a Baldur's Gate III-style sequel. It's a--and I'm stealing shamelessly from Borderlands here--pre-sequel. (Side note: The new expansion only works with Baldur's Gate Enhanced Edition, so no need to go digging out your original 1998 Baldur's Gate CD-ROMs.)
And it's sounding great, from what few details have been released. The original cast of characters is back, there's a new class (shaman), four new companions, and all the art is done in the Infinity Engine aesthetic. Also, twenty-five hours is a respectable heft--that's almost as long as spiritual successor Pillars of Eternity, which is its own standalone game--and you can still import your Siege of Dragonspear save into Baldur's Gate II.
No release date yet, but...well, it already took fifteen years. I think we can wait a bit longer. We'll be sure to update you when we know more.