Why a sample comprising (exclusively) silicon valley start-ups can be considered a random sample.

regarding the second question: how the (exclusively) Silicon Valley start-ups sample is different from the canteen students sample (from the video)? If the population comprises all the US start-ups how can a sample composed of exclusively Silicon Valley ones can be considered random? I believe this would be the case (a random sample) only if we considered the Silicon Valley start-ups as the population.