Everything clearly means everything, everything that has been once, thousands of years ago, and what is today might very well be lost in thousands of years from now on. Guess it more means that mankind does not really develop, doesn't build on it's achievements, just toss one thing and start another from time to time.
However I wouldn't agree fully with that statement, as we still know some little bits of what the people once knew, and are also somehow able to make educated guesses (from archaeology) - still, much is lost indeed.