what he is saying is that, if the rock was already cured, that means it's already got the beneficial bacteria colonized on it and when ammonia/nitrite are present, they will be dealt with faster than you can get out the test kit and register them. This just means the cycle probably happened the moment you stuck the rock in and before you got the testing done for the first time.
The ammonia being at 0.5ppm could just be your test kit. My AP brand test kit always shows at a minimum .025 on the color chart, weather there is ammonia or not, just inaccurate that way.
Now all that being said, if there is no bioload being added to the tank, such as dieoff from uncured rock, you wil not see ammonia or nitrite or nitrates. You need something "filthing" up the water for the rock to actual cycle anything. I would keep testing for ammonia and nitrite, nitrates will show up soon enough, especially after you start adding livestock. Maybe get some dead jumbo shrimp (1 piece) and stick it in the tank, that'll get a cycle started for sure, or use a freshwater molly for a couple weeks (acclimate for about 4-6 hours to get him in full saltwater).
Since there is some doubt as to weather you had a cycle, just take it nice and slow and don't go add 30 fish tommorrow.
