IFS system

So I recently got out of a match where I was flamed super hard, Called a "retard" and "trash" by my top laner about 4-5 times. I just would like to know how the ifs system has not picked up on this harassment. After the third time I muted him, but when I mute people it usually already means im tilted from you. In the after game lobby, he just kept saying that same stuff.

How does the ifs system not pick up on language like this? is the word "retard" not as offensive as being called something else?