That's fair, thanks. Whenever I start looking at bit-level stuff I start wanting to make sure it's rock solid, a habit drilled into me from my professors.

It's 7 kyu kata, I don't think there's a need to make it more complicated than it already is

Yes, I don't make suggestions or comments on katas often.
Since the function allows input of two 64 bit numbers, good practice is to be prepared for a result that may be longer than the inputs.
There is a test for adding two zeroes; there should be a test for the upper bounds as well, in my opinion.

Is this a question? The short answer is probably "no".

The long answer is that tests are specifically written to avoid 64 bit overflow, so the answer is probably "no".

In C++, since input is uint64_t, should tests include edge case for result that is 65 bits long, e.g. add_binary(UINT64_MAX + UINT64_MAX)?