Getting to rifling while the AI is defending with longbows/muskets doesn't mean you're massively ahead on tech, especially if you are factoring in the liberalism launchpad.
Really, it's more an indication of Cavalry being overpowered. Well, not really overpowered in themselves, but there not being an appropriate pre-period counter-unit. There is no unit before Riflemen that can stand up to Cavalry in a 1-on-1 match. That's a game balance mistake. There really should be some sort of str 9 upgrade to Pikemen (or something similar) to provide a decent counter-Cavalry unit. This lack is, I think, what allows for the dominance of Cavalry if you manage to get them first.
Of course, I think the same thing is true for Riflemen, the power increase compared to previous units is incredible. You generally jump from a str 8 city attacker to a str 14 city attacker, a 75% increase. And while you may expect slightly stronger defenders in Musketmen, Longbows can't be upgraded to them, which means you have to build new ones, and they don't get the inherent 25% CD bonus that Longbows get. In other words, unless you're fighting a Protective Civ with hill cities, if you're first to Riflemen, you have a tremendous advantage.
I think that Firaxis did a decent job of introducing some late-game counter units, but this middle-game area is the one part that I think is still lacking.
Bh