We have all got very accustomed to the notion that we can put content on a website and it will stay there forever, permanently available, as if that site somehow has an obligation to look after it. But they don't.
It sucks, and there will be a lot of stuff lost, but it's also good to have a reminder that if there's data you really care about, you need to look after that data yourself.
Did you genuinely interpret that as a child to mean "If you put something on the Internet will be safe forever"?
As I'm sure you are now aware as an adult, the intended meaning is very much "If you put something on the Internet which is embarrassing to you or damaging to your reputation, then it will be around forever"
It's a warning that the things you don't want to stick around could end up being precisely the things which do.