I do think Albert, at least, has faced that type of pitching.  Hell, knowing how competitive he is, maybe he had his wife pitch to him for a few weeks before the episode taped. :P  It's no secret that baseball players play/played softball, too.  So they've seen that type of pitching before.

Now, we don't know what weight/length bats the players used.  Perhaps they used a lighter or shorter bat when facing Finch.  The fast-pitch softball is also larger than a baseball, though smaller than a slow-pitch softball.  They have a bigger target to hit than with the equivalent baseball fastball.  

I don't have footage of the TWIB episodes, so I don't know Jennie Finch's exact release point, but I do know that in fastpitch softball, the ball has to be released at or below the hips.  I'd like to see the TWIB footage to see whether Finch altered her release point when facing the MLB hitters, perhaps to compensate for their height.

Look, the whole thing was just kind of cool and fun.  She's obviously talented, as I doubt many (if ANY) other softball pitchers would be able to strike out a bunch of MLB hitters.  You can discount it by saying the mound was different, etc, and I've got just as much to say that the MLB players have advantages, too.  Such as, with the riseball that she threw, the MLB players are taller than her usual competition.  Thus, they have a height advantage, creating a different swing plane for them, one that could be advantageous as the riseball wouldn't appear to rise as much to 6'3" Albert Pujols as it would to 5'6" David Eckstein.