Sure. There were 20 items in the bucket with the same hash, we wanted to add one more.

So we grow the top level hash table and we put all of the twenty items in the same new bucket, because they have the same hash, and then we... oh, we don't have enough room. Try again? (The code presented does). Too bad, that can't help.

In your understanding, why does this not happen? That's the question. How do this code ensure this can't happen? If it doesn't, if it's just hoping to get lucky the code must not be used.