Early versions of iOS did this too. You’d tap an app icon and the app would appear instantly in its prior state … but it was just a saved screenshot that iOS displayed while the app was actually loading.
Really it still does, in a way. The ”apps” in the app switcher work that way. Even if the app is still live in memory, it’s not rendering to the switcher all the time. And if it was killed, then the pic is all that’s left.