My point is based on experience. Yes, it's probably too much to ask to test all 2^64 cases, but I've been dealing with code in production that was failing precisely because some values lead to code paths that gave incorrect results.

In this particular case, I would have tested 2^N where N>32 random, unique values, at a minimum.

Of course, it depends on who the customer is. Some customers can tolerate a few bugs here and there; and some would incur million dollar losses.