@maxal from Wikipedia: Flintlock is a general term for any firearm that uses a flint striking ignition mechanism. The term may also apply to a particular form of the mechanism itself, also known as the true flintlock, that was introduced in the early 17thcentury, and rapidly replaced earlier firearm-ignition technologies, such as the matchlock, the wheellock, and the earlier flintlock mechanisms.
So flintlock fantasy means fantasy with gunpowder and guns basically. The term serves the purpose of alliteration and approximate technology level.