There is nothing illegal about a monopoly. A monopoly that uses it's position to prevent competition is illegal. Giving DDG a very valuable domain name (given their name) absolutely does what you're saying it doesn't.

How is not giving the domain name to DDG using their monopoly to suppress competitors? This is not equivalent to say, removing DDG from their search results.

I don't think that's what they're saying. The point isn't that withholding the domain name is the end of the world, the point is that handing it over is a small token that Google can use to claim that they aren't anti-competitive. I don't know what my stance is on that interpretation, but that seems to be (to me) to be the intended interpretation.

Does that make sense?

That's not what they're saying. If/when the feds come after Google for (most likely unrelated) monopolistic suppression, Google can say "we don't suppress our competitors--look how we helped out DDG!"