"More work is still needed to define policies that would allow array users and manufacturers to detect unusually disk failure rates and take the appropriate actions before any data loss takes place." - Last line in the conclusion.
This implies that not all the spare drives are active and ready to go all the time and that some/most would be kept powered down as cold spares. Of course this same guy is likely to get another paper done where he examines the cost to run the array and how many drives could be left cold and still achieve the 5-9s reliability. Heck, if the software managing the drives is smart, it would rotate active/spare drives in and out, working them in quickly to get them all past the 'first 18 months high failure' rate to the sweet spot, then swap in and out over the lifespan of the array to enable the array to be at highest reliability for longer.
Hrmm, maybe I should look at building such an algorithm, a quick google search doesn't turn any such systems up.