Because a plane just flew over their house

Naah it's just them assuring you that they have flushed their toilet today. It's good internet manners.

It's a community for making fun of people who completely missed the joke. They are telling you that you are being dumb for not understanding that it's a joke.

It's a long historied phrase only typed in response to weird trolls as a way of telling them that no one cares that they spend an hour every day just seething and posting about their Linux rage