Throne of Bhaal was an expansion.Mask of the Betrayer was an expansion.
See, I've played games too!
Disclaimer: I'm completely fine with White March being like TotSC and not ToB.
An expansion adds new areas. A sequel continues the adventure.
Not sure what point you are trying to make.....
That's pretty much just an arbitrary distinction. While Throne of Bhaal and Mask of the Betrayer do continue the adventure, they're not sequels - they aren't Baldur's Gate 3 or Neverwinter Nights 3, they're still (or were still) advertised, listed and sold as expansions because that's what they are.
A more accurate way to define them is that an expansion is additional content for an existing game, while a sequel is a new game entirely.