As for the 'all marbles are pushed simultaneously': a key characteristic of useful transistors is that they operate as amplifiers[1]. This is possible because effectively, every transistor has its own power source. As such, on each clock cycle, then can all get a simultaneous push from their power sources.
BTW, I'm not conflating the speeds of light and sound. If you bang on one side of an iron bar, a wave travels through the material, quickly compressing and decompressing the bar where the wave passes. Such waves travel at the speed of sound in that material, no at the speed of light. That makes sense, because sound is nothing more than the physical modulation of the density of a medium, most commonly air.
You're not, the person you're replying to was. I find it odd that you would object to an analogy where the signal propagates through the marbles at the speed of light (i.e. 'instantly'), and offer up instead one where each marble moves simultaneously. If the exit event is simultaneous to the entry of the other marble, they are separated by a spacelike interval despite the causal relationship between the two events. This violates relativity.
On the other hand, in the original analogy the two events are separated by a lightlike interval, which while impossible in the case of marbles, at least does not violate relativity when the two events are causally linked. This is what I meant when I said the conflation of the speed of sound and the speed of light is not such a big deal, in this case.
[1] http://en.wikipedia.org/wiki/Transistor#Simplified_operation
BTW, I'm not conflating the speeds of light and sound. If you bang on one side of an iron bar, a wave travels through the material, quickly compressing and decompressing the bar where the wave passes. Such waves travel at the speed of sound in that material, no at the speed of light. That makes sense, because sound is nothing more than the physical modulation of the density of a medium, most commonly air.