First... that's a lovely drabble. Not all the monsters were left behind, indeed.

Second... ::prepares to admit to absurd levels of geekery::

If you pause the ep when the Doctor is seeing history change in the form of the news articles after Adelaide commits suicide, you can see some of the news blurbs about Mia and Yuri. They tell the world about how Adelaide saves humanity and a mysterious alien named the Doctor brought them back to Earth.