We don't know the full story behind that tweet. Inverting a binary tree is not exactly a difficult problem. They could have even spoon fed the author with "Ok,

We don't know the full story behind that tweet. Inverting a binary tree is not exactly a difficult problem. They could have even spoon fed the author with "Ok, here's the code to traverse a binary tree. This is what it means to invert a subtree. How would you invert the rest of the tree?"