Have you overlooked the word unaided
in the quote?
What does the number of grains have to do with anything?
And on a sandy beach, there is a chance that there will be "aids" to make the hopping
out of the box possible. The wind could pry open the matchbox. A smoker could pick it up in search of matches and empty it out in frustration. A crab could come...
The beach context teems with possibilities that make the hopping practically possible. However, lock away the matchbox in an empty vault and chances are that there will be no hopping.
If we think of quantum particles and acknowledge that they behave somewhat like a particle and somewhat like a wave in a probabilistic frame, we will find that within the inner and outer cluster of peaks and troughs, the waves cancel each other out in certain points. This will quickly take place as we make the measurement, because, things on a smaller scale secretly
play out all scenarios before settling on the most likely outcome. Heisenberg's Uncertainty principle hinges on this.
We must also remember that, on a quantum level, it is apparent that nothing is at rest, even those particles which are not effected upon by force - this is where Newton's laws (which although logical and very applicable on a macro scale) are thrown out of the window. The more precisely you know the position of a particle at some instant, the less well you know how fast it is moving and therefore where it will be sometime later.
Now, we live in a quantum universe, and, a grain of sand is a relatively ginormous object in the quantum realm. It is so huge that it no longer appears to obey quantum laws and conforms to classical physics. However, it is made up of those tiny packets of energy that move and jump all the time. The bigger the object gets, the more time is incremented in their unaided motion. Make no mistake about it, according to quantum theory, the grain is moving unaided (because, like everything else, it stems from the quantum realm) but it is too big for any noticeable movement and its properties can easily be measured. This lies at the heart of quantum physics. It is encompassed in Heisenberg's breakthrough formulae. This is how things really work. Newton was not wrong, but, his picture was incomplete.
Imagine a grain of sand inside a 3cm-length matchbox. Physicists can calculate how long it will take for the grain to hop to the confines of a 4cm radius - beyond the matchbox (!) - unaided. Do you know how long it would take? Over a thousand times the current age of the universe! Meaning it won't happen! You might as well say that the grain is not moving and be content with that.
Quantum mechanically speaking, not impossible but improbable.
Hence where Cox is coming from with the matchbox and grain quote. The scenario proposed by quantum mechanics is hard to swallow and often hard to understand, but, that is the nature of the universe. Things do really happen that way and this is the only way we can explain what happens in certain experiments. It is just a matter of people getting past their prejudices. The Newtonian era is long gone and there have been many breakthroughs - not that the majority of laymen are aware, though.
To bring it back to the main topic, as I feel that we have gone off on a tangent, anyone who thinks lucid dreaming is a ridiculous notion is far behind. People experience them. Most importantly, where have these people been for the past 40-odd years - which is how long it's been since the phenomenon was scientifically recognised as fact for the first time?