The message links to a filter that includes a long list of exceptions like "@@||www.google.com^$document" and "@@||maps.google.com^$document".
Here's a screenshot of the Google whitelist filter (it actually includes more domains):
This is pretty weird, considering that the ads were not blocked, as you can see in the screenshot. Google ads are not blocked by default in AdBlock Plus, since they're considered "acceptable ads". Some sites have speculated that Google paid AdBlock Plus to whitelist its ads, but that's not very clear.
Google Discovery also reported about this back in May, so it looks like the experiment is limited to Google Brazil, at least for now.
{ Thanks, Marcos Alexandre. }