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I suspect they plan on recovering the power with a generator. So the input shaft to the generator has to rotate. With a deeper hole, you get more 'clicks' on the generator so you can produce power for a longer period of time before having to pull the weight back up.

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Does it really matter?

You should be able to get millions of rotations from a single centimetre hole, given that the mass is large enough and you have a good gear ratio..

I suppose they plan on using a transmission mechanism that will turn one meter down to as many turns as you want anyway, so I guess the depth of the hole is at least partially determined by how good a transmission mechanism you can afford. But since they explicitly say the hole will be the most expensive part, I'm not so sure.

You're losing torque with every gear reduction, and I believe that current out of a generator is a function of torque. So there's some happy medium between hole depth & transmission ratio for the amount of power they want to generate.

You're certainly losing torque with the gearing but that's not a problem. The problem is losing energy. Energy loss can be made arbitrarily small at increasing cost to build the transmission.

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