It looks like they are currently using Chrome 41 (https://developers.google.com/search/docs/guides/rendering) when rendering pages. I agree that with all of the work on headless Chrome, they should move towards using headless Chrome in the future.
I wonder if all. the. security. patches. from every subsequent Chrome milestone regularly get backported to M41?
Obviously it's sandboxed to the hit. Poking the sandbox and seeing what it was made of would be VERY interesting though.