Even though a break-in on a hydraulic cam isn't completely necessary...the reason why the instructions say that is that at idle there may not be enough oil to "cover" the cam and lube it up nice and that way if you keep the revs up (around 2000 w/ small variances in revs) it assures good oil flow...
Edit: You should clean off your cam to begin with a nice solvent...
There is a protective coating on the cam to prevent oxidization during shipping. You WILL need to run at fast idle to wear it off otherwise it can and will cause more agressive wear to the cam or lifters.