Classical bioisosteres
I think you are right. Grimm's law in its original version (1925) should not apply to the $\ce{-CF3}$ , $\ce{-CN}$ pair. This "law" is very restrictive and has only historical significance. To be honest I could not recall its statement and I have never come across it in the last 10 years or so that I work in the field. The modern definition of classical bioisosteres (as opposed to non-classical bioisosteres like the $\ce{-CO2H}$, tetrazole pair) has been extended exactly for this reason, to include functional group pairs with similar physiochemical properties that do not follow Grimm's law or any other classification of less general scope.