Are you reading "to a first approximation" as "to one significant figure"? I thought it meant something like "using the lowest order function which is an accurate fit to the data". So, to a first approximation, pi is roughly 22/7, and to a first approximation, the distance to Earth's horizon in kilometers is 3.57 times the square root of the height above sea level in meters.

To say that "to a first approximation, all biologists believe in evolution" is, by this definition, to say that the fraction of biologists that don't is so small that it is not easily measured. I believe this to be the case because that fraction is so small that it is significantly affected by the choice of definition of "biologist".