Based on what I've seen, fellow Googlers use Macs as glorified ssh terminals + chrome machines. Most Google code isn't allowed on laptops per security policy, and most wouldn't build there anyway. I also think lots of people have Macs because, before the Pixel, they were the only laptop offered internally that qualified as a fashion symbol, and the only option with a Retina display.
Also, many people who use Google tools also have macbooks (e.g. all the Googlers I know), they have to use the tools with something.