You can make a picture of a screen with camera. I always do that with apps that restrict screenshots.

Sure sure, but it's one extra step that may not always be available.

I might be this missing a point here. What is the reason to block screenshots? If we want to save space, blocking screenshots is irrelevant. If we want sender to have a guarantee that message is not preserved then we can't do that either because screen is visible. I can only see that as a marketing trick for unsophisticated user.