Since you understand politics so well you understand Mayor Breed can't unilaterally write housing policy. She has to work with the Board of Supervisors. The Board is dominated by a group of politicians that aren't really YIMBY supported as far as I know.
Since your reading comprehension is so high, you'll agree then that supporting yimby groups won't be sufficient to generate the credible threat that the original comment suggests will be sufficient to dissuade investors
