GP's reasoning is probably: the only plausible way to detect human civilization (and not just our planet, which is already hard enough from a different star) is by it's radio signature, and the oldest radio emissions is from 100 years ago, give or take. Anyone standing outside of a sphere whose center is earth and radius is 100 light year hadn't been reached by our oldest detectable-from-space civilization artifacts yet.

It strikes me as a pretty intuitive arguemnt to make and true by default, to show it wrong you have to find or somehow plausibly argue for the existence of some physical medium or process that human civilization (or even earth life in general) emit or modulate in unique, detectable-from-space ways other than electromagnetic waves and that we have been doing this since before radio.