I'm wondering if Google is somehow, in some way, using the rendering data generated by the Chrome clients and/or Android to aid with processing power it takes to index everything.
More likely they're getting lots of data from analytics users for a great number of sites as it is, and only really need to do custom renders for load time analysis for some sites, and not necessarily all pages... to a larger extent, I'm pretty sure they could have an optimized rendering pipeline for a headless chrome that actually works better than, by comparison, phantomjs.