The Dahrīs are well accounted for and discussed quite a bit.
"One who believes in the eternity of matter, and asserts that the duration of this world is from eternity, and denies the Day of Resurrection and Judgement; an Atheist (Ghiyasu ‘l-I-ghat, in loco.)" https://www.juancole.com/library/dictionary-of-islam-hughes/dahri
I am just adding that instead of being agnostic, it seems likely that some denied God altogether based on this Verse. I don't think it's a big leap at all. I'm not suggesting that there was a large atheistic trend, but there were enough Dahris that the Quran decided to deal with them in several places and respond to someone postulating that there isn't a Creator.
Additionally, if we take an atheist to be someone that doesn't believe in God, not neccesarily someone that postulates that God doesn't exist, that essentially describes the Dahris.