There's no way lawn mower produces 20x the carbon dioxide a car does, in fact it would be considerably less than a car since they use much less gas. Probably whoever said that was referring to other pollutants, since most mowers don't have catalytic converters. But, for CO2, cats don't help.
To the person who said the goats will produce the same amount of methane, regardless, take a second to think about it. If you're a goat grower (or whatever they are called), and now your goats are committed to Google for a week at a time, you're now in a situation where you're goat poor, and you need more. So, now you're breeding for more of these beasts - you're demand has gone up and now you need to increase your available goats. It's not like these things are strays and you rescue them from the streets, they are deliberately bred, and that is based on demand. So you end up with more of these beasts, and therefore more methane. Also, think of all the metal cans that could have been recycles via the goats, that now require incineration 😛 .
And as someone else said, the amount of gasoline you have to use to bring these things to "work" is pretty expensive. It's good PR, for sure, but if you look any deeper, it's disingenous. But, still, if I were working for Google, I'd definitely enjoy seeing goats out there munching down on the weeds. It's good for moral, and employee moral is very important. But, I'm not sure it's good for global warming. I'm pretty sure it isn't.