Quran went into writing well before the Hadith were analyzed and collected for authenticity.
I am sure you know that Quran was first quoted (by Prophet Mohammad) and later narrated (by sahaba) through chain of narrations and even later on complied, pretty much the same as Bukhari and Muslim were compiled. Same people, smilar method. Today after 1500 yrs, all of a sudden we realized that we can take one book word by word but reject the other although people involved were the same.
Quran is nothing but hadees, later told by Prophet to Sahaba that this was kalamullah so sahaba were able to separate the words of Nabi from the words of Allah.