'We don't say these giant humans are a breed of humans, but if we mated these giants together then eventually over time these humans would all be giants, and would inherit the same disease. But we don't currently consider giant humans to be a "race" or a "breed", so why is that? '

You've hit on a really fundamental question in biology. Those two men were individual mutations within a population (most general term). When does a population become a species? By definition, when that population can no longer interbreed with other neighboring populations.

But it's a dynamic process. All human ethnicities (to use your word) can interbreed, as can all dog breeds. We call dogs, wolves, and coyotes separate species, and under normal ecological conditions they wouldn't interbreed, but they can and sometimes do (which is the exact problem of humans wanting to label transitional states in a process as separate stable things that you describe, just at a different level that most of us agree on and don't think about any more).

So yes, the pop culture use of the word race is a grossly simplified, socially constructed set of arbitrary categories, which are only loosely coupled to the seething, dynamic genetic reality. But even our species definition is kind of the same thing, only somewhat more reliable because of the condition we put on it that "they must not interbreed successfully." Nature doesn't respect our labels and boundaries.

So dog breeds are not different in kind from human ethnicities because humans directed the process. They're further along in the same dynamic process of natural selection (which often has equally arbitrary and weird outcomes) because humans sped it up. Artificial selection is just natural selection happening really fast.