From there you can say that works fine with pen and paper but why might it go wrong in a program? So how do you avoid integer overflow? If the signs are the same then use subtract first else use addition first. After we have done that. After this step assume I have returned to the main flow. How do I check if I should use addition or subtraction? If a is bigger than b I must have added so use subtraction. Writing the algorithm this way is trickier than xor. The problems to be solved are to do with understanding concepts
Next you have the xor solution but more important than getting the answer to that might be to see if they can see that the two functions are like encrypt and decrypt where a is the message and b is a key and a + b is the ciphertext c - so that f(m,k) is encrypt and g(c,k) gives the message and g(c,m) gives the key. If they have had any exposure to cryptography then xor should present itself.
If they give xor straight off then you can work backwards.
Those tricks might be valuable in your workplace, and may make excellent interview questions for you, but they are a very very different type of question than fizzbuzz, at least in purpose, even though they may look superficially similar. In a fizzbuzz type question, you are not looking for brilliance, you are looking to just weed out the people who can't.
"Swapping two variables without a temp" is a bad question, because it depends on knowing the trick.
"Find the 2nd-highest element of this list" is a basic question, and I can change it up it a zillion ways such that anyone trying to just learn all possible permutations will end up doing way more work than someone who can just figure it out on the fly.