It seems to be nothing more than a conveniently not-wordy designation for 'drow that don't follow Lolth' in the game. That being said, it is inconsistent. Minthara isn't one, even though she doesn't follow the spider queen.
There's also the issue that for some bizarre reason Larian decided that drow eye color changes when they renounce Lolth, and the reverse as well. Doesn't exist anywhere else in any preceding novels or sourcebooks-completely unprecedented, and tbh....kinda uncomfortable IMO. Again, pretty inconsistent. There are actuall very few drow in-game you can meet that follow Lolth, but some are arbitrarily 'Lolthsworn' with the red eyes, while others are 'Seldarine' With blue/purple eyes (supposed to be rare eye colors in previous material, but near everywhere in BG III)
Seldarine strictly refers to the elven pantheon outside Lloth and the betrayers. Minthara is not Seldarine because she doesn't follow the elven pantheon. She's a brainwashed captured Lloth drow.
You also find out in Moonrise Towers that the drow who supposedly abandoned Lloth did not in fact abandon her. Ketheric with his advisor trapped drow by tadpoling drow scouts, sending them to whisper of the Absolute in the Underdark, and the angry Matrons would send Lloth loyal hunting parties after them. Ketheric reasoned it was worth it to sacrifice one tadpoled drow to ambus the entire hunting party and convert them via tadpole, as drow made natural leaders to the chaff in the army. You can find this in the books/letters around Zrell, Balthazar, and Ketheric's chambers in Moonrise, alongside with missives mentioning their alliance and plans with Gortash and Orin.
Minthara in essence is a drow that was captured in a hunting party like all drow in the Moonrise Towers, and their memories were manipulated by the tadpole.