I think the slogan was meant to be provocative but unfortunately it has been misinterpreted more often than not.

For example, imagine you're working with a 3rd party API and, according to the documentation, it is supposed to return responses in a certain format. What if suddenly that API stops working? Or what if the format changes?

You could write code to handle that "what if" scenario, but then trying to handle every hypothetical your code becomes bloated, more complicated, and hard to understand.

So in these cases, you accept that the system will crash. But to ensure reliability, you don't want to bring down the whole system. So there are primitives that let you control the blast radius of the crash if something unexpected happens.

Let it crash does not mean you skip validating user input. Those are issues that you expect to happen. You handle those just as you would in any programming language.