But a lot of it is also just finding a place with a green power plant they can buy energy from, which isn't really helping anyone (power plants generally run at capacity; someone else was going to use that energy and it doesn't matter who does it).

In theory it does kind of matter in the sense that if every company were committed to using green power in the same way that it would necessitate more such plants being built. If another company is so committed and Google is using all the green power, there'd have to be more green power plants built to satisfy the demand. And Google doesn't really have much control over what kind of power other companies prefer to use.

But yeah, ultimately that's still a small thing