Why are these two lines of code not equivalent?

Chang, the challenge is wrong. Good eyes! They do exactly the same thing. You should report the error.

Consider this code. If you put it in an Xcode playground, it will display what I've shown in the comments:

var dict = [1: "Hello", 2: "Good Bye", 3: "X"]  //[2: "Good Bye", 3: "X", 1: "Hello"]
dict.updateValue("yetAnotherValue", forKey: 3) //"X"
dict  //[2: "Good Bye", 3: "yetAnotherValue", 1: "Hello"]
dict[3] = "yetAnotherValue2"  //"yetAnotherValue2"
dict //[2: "Good Bye", 3: "yetAnotherValue2", 1: "Hello"]

Re what they do, both are setting the value of the pair with the key of 3 to a new value. So the first changes the pair 3: "X" to 3: "yetAnotherValue", and the second changes 3: "yetAnotherValue" to 3: "yetAnotherValue2"

P.S., the second is not returning the value at the third positions ( [3] ) from the dict dictionary, This line of code would do that:

let value = dict[3]

Thanks for the comments, David and JCorum! Guess we all agree the results to dictionary dict are the same. From that angle, these two lines of code should be consider as equivalent, as there is no other side effect.

This question should have been updated by now... very confusing to be told you're wrong for something that's clearly right.