This is just quoting the article under the "Algorithm LG: (Logorithm method)" section :

> In FORTRAN the algorithm is best programmed directly as
> $X=-ALOG(REGOL(IR))$ avoiding any subprogram call. The performance was
> 504 $\mu$sec per sample. Of this time 361 $\mu$sec was taken up by the
> manufacturer's logarithm routine and 105 $\mu$sec by the generator for
> REGOL of the uniformly distributed variable $u$. Therefore there was
> no point in trying to improve speed by writing an assembler function
> which would have to use the same two subprograms.

So it looks like the authors opted for other methods to avoid this "manufacturer" limitation of slow logorithms. Perhaps this question is then best moved to stackoverflow where someone with knowledge on the guts of R can comment.