My hunch is that Google has a oncology/immunology platform which they're not talking about but trying to actively develop in order to help fuel Calico's pipeline and maybe sell off some other generated IP. There's almost certainly a "big data" angle as well, potentially describing the polymorphisms in majorhistocompatability (MHC) molecules between people. Having a large dataset of this kind of information would provide a lot of predictive power for infectious disease resistance and also disease progression.
Google is new to this space and enjoys young talent, so I can tell that their approach will be to hire the people graduating from the top biomedical PhD programs. This is a hiring market that they can easily dominate by offering salaries of 50k-90k, though they may bump this range up to ensure loyalty. Nobody else will offer a better game in town to fresh PhDs in the current (extremely hostile benefits-wise) science employment environment. Google gets in on the ground level, and the fresh PhDs avoid wasting 5 years on their postdoc-ing. Once the platform is established using the young talent, Google will spend a lot of money bringing in older mid-level people from prestigious organizations to mainstream-ize development and provide more credibility.