Of all the sources outside of Google, I'd say XKCD is likely to be closer than most. Even if you ignore the fact that Randall (XKCD) spends a good amount of his time estimating things in a rigorous and scientific way that would mean his estimates are actually pretty good, he has the exact background that means people at Google would take his call if he picked up the phone to muse upon the question of how many machines Google has. They might not give specifics, but helping would be a pretty cool thing to do so they'd probably do it.

Do you imagine that the error bars would be so large as to significantly change the order of magnitude? I don't.

Anyway, it's an offhanded comment and its precision is almost completely irrelevant to the article. I don't fault the author for not spending too much time on it.

Let's just say that when Xkcd made that estimate, it seemed pretty good to a lot of people.