The dudes commenting are idiots. They’re saying “can you believe it? Everyone walks past him, and they don’t see him! I would have not believed it if I didn’t see it. It’s so weird!”

That is, to them (the commentators), it’s not that the people don’t suspect of the “homeless dude lying down.” To them, nobody is actually able to see that there is a human being lying there, as if he was invisible.