So two things that are not accounted for here:

1. This covers only brute force attacks, meaning you try every different combination. Shor’s Algorithm exploits patterns in RSA keys and is much, much more efficient than brute force.
2. There actually is an algorithm (Grover’s search algorithm) that can speed up brute force search. However, this speedup is only quadratic, so brute forcing something like a 256 bit key is still infeasible. The discrepancy is quantum information doesn’t flow the same way that classical information does. A related concept is the idea of reversible classical computing: this derivation relies on the assumption that you change set a bit, thereby erasing the information of what that bit was before. If your operation doesn’t erase that information (e.g. if it’s specifically a bit flip, you know what the original bit was, it’s the opposite), then this argument about the minimum required energy falls apart. Most operations in a quantum computer are inherently reversible.