You are right in a laboratory way of let's-assume-perfect-adversary, and you're not in a how-it-really-works way. I simply have my own RL exp in a bad repressive country on this topic, and my claim that e.g. hidden volume works most of the time. Like any tech it will stop being useful eventually, but it has been saving people for quite some time.
Why is that is a long story. And it includes both general inefficiency of state apparatus, but also the fact that repressive systems aren't optimized for getting evidence (or just correct information) in most cases.