The wait is so the tank has time to cycle and mature enough to handle the new load (bio load) of the octopus.
Your live rock has bacteria on it and it has to have time to multiply after a cycle that way it will beable to help clean the water.
If you have uncured rock to start with then you have to cycle it. Cycleing is the die off on the rock .
Study about the "Nitrogen Cycle" if you arn't sure to what it is. Every aquarist must comprehend fully the process of biological filtration.
That being said, from what i've read and been told, having a "cycling fish" such as a hardy damsel (or some even use an eel) helps the bacteria cycle as the bacteria that you want in the tank "feeds"--so to speak--off the ammonia made by the cycling fish, thus helping the process.
I would have to imagine, however, the the waste produced by an octo would be an ammonia overkill on a new system.
LOL had to read that several times to get what you mean but Octopussy is right in what was said.. (if i understood that right :) )... in that a cycling fish produces the initial ammonia needed to start off a colony of bacteria in the filter and without it, a tank would never cycle or have enough bacteria to support adding an octopus without the octopus producing too much ammonia so that the bacteria couldnt handle it and poison the octo.
so cycling is the name given to establishing a colony of bacteria large enough to cope with an octopus when it arrives... In that case... all cycling fish should be removed at the same time as adding your octo or you will be adding a bigger bioload with not enough bacteria to cope with with the new ammount of ammonia being produced. Also remember that an octopus produces three times more ammonia than a fish of equal mass.
If i was cycling a tank for a bimac at a 2" mantle i would cycle with approx 10 black mollies and remove them the moment before octo went into tank... hopefully that would mean no ammonia spike and no problem for octo...