You really can spend a lot of time just messing around. Stopping and listening to broadcasts, messing around with throwing pop cans at combine guards, throwing TVs out of windows, etc. It should not be hard to imagine somebody taking a bit longer to get through the game, due to getting lost, dying more often, a more conservative strategy, or whatever, coupled with several hours of messing around over the course of the game, and of course the playtime will vary wildly.
For instance, there is a room during the hovercraft course with lots of grenade boxes around. You can simply storm that place with machine guns and punch through it easily. Or you can take it on with grendades alone, and it can take much longer.
I do think its fair to say you missed out if you only completed it in 9 hours. Even if you did find all of the dialog(and how will you know if you didn't?), the shorter playtime means you spent less time messing around, and that type of messing around is part of the game.
How often did you tackle a section with inly 2 hp remaining, resolved to not load a save any further back, and instead play a section a half-dozen times to figure out a method to do it without getting scratched? How much time did you spend trying to tackle ravenholm with the gravity gun alone? At a certain point, it is fair to say that you did not physically play the game long enough to experience everything it has to offer. Its not just about the content they put in the game, its the interesting experiences you have from exploring the game in-depth.