"Obviously, those who want to avoid compression can just take screenshots and then save them in a lossless image format of their choice" This might be a bit of an assumption. Could it be that it saved the jpeg first and is showing that, not a lossless result? I suspect it's showing the jpeg as that's what I've seen in many other applications.
The recently released text-to-image generator by Facebook's and Instagram's parent company was trained using 1.1 billion public images from the two websites. Unfortunately, this fun online resource that is powered by artificial intelligence is only available in the US for the time being.