Cured live rock will definately have nitrifying bacteria on it, that's a given.
Will it have enough to process the ammonia produced by a fish? It might...it might not. If it was just taken from a functioning setup it probably will but it's not a sure thing. If you put one of your pets in there and there isn't enough bacteria yet you could experience an ammonia spike or worse, a Nitrite spike. This will cause the fish added stress and it will be suseptible to illness like Ick, could cause brain damage, it might even kill it.
If you produce an ammonia level you will allow the nitrifying bacteria to reproduce and when the ammonia and Nitrite go back to zero then you can be SURE that you have enough bacteria to handle the fish waste. Otherwise you're just guessing. If you add an ammonia source and no ammonia is detected then you can also be sure that you have the required bacteria population.
I just question the judgement of people who actually prefer to cause suffering on a fish when it's just as effective to use something already dead.