Even assuming his drunken source was accurate and truthful, there are other explanations for how Google could access the source's email.
This event happened "a few years ago", when Yahoo, Hotmail and AOL weren't protecting their email with SSL. Google could easily watch unencrypted traffic crossing its internal network and flag sensitive communications.
If they're not doing this, they should be. They don't have to read Arrington's Gmail to get his source's unencrypted communications with a non-Gmail provider, as long as his source was using a Google computer or a Google network.
- choice A: Google is logging all unencrypted communications from their staff (a rather vast amount of information altogether, I suspect, given how Google employees throw data around), in order to be able to go back retrospectively and wade through it to find leakers
- choice B: Google grepped through Gmail to find the leaker, which they have a complete legal right to do and has a marginal cost of zero.