Not really a spoiler, the author just did not want to draw another page to explain this but - from what I remember - the WN explains at this point that Nie Lie reached Heaven Spirit Root in an instant as opposed to gradually have the roots progress each rank and if he didn't pull out his hand very quickly the roots would have split one more time (9-Grade Heaven Spirit Root), turn black and jump into a higher, unknown rank. Even though the examiners failed to see, Nie Lie did notice that the 9th root was already starting to form and the entire root system was already turning black as seen in the manga.