"intentionally" letting your eyes defocus is not a simple task for most people. Because most people use their eyes normally, they can't do things that are unusual. Look at how many people struggle with Magic Eye stereograms. Squinting on the other hand is something that is simple to do.
Downsizing is effectively low pass filtering, so that's expected. Any scheme that transmits different messages in different frequency bands is going to be susceptible to a similar attack.
It's only when I squint hard that I can see "HAPPY HUMAN".