I think the factor of a billion relates to the expectation for boron fuel with spherical compression. See for example their latest paper, page three, right colu

I think the factor of a billion relates to the expectation for boron fuel with spherical compression. See for example their latest paper, page three, right column. They do seem to be making quantitative predictions based on simulations. I don't think I've seen them boil it down to a simple scaling law though.