To answer your question another way, yes, you understand correctly. The presence of chloramine can cause inaccurate ammonia test readings. That would explain why your tank was registering an ammonia reading even before you used Prime. For those reading that don't know, chloramine is chlorine + ammonia combined. The test reacts to the ammonia present in chloramine.
Will the ammonia be taken up if there is no bacteria present? Soon after an ammonia source is introduced, nitrifying bacteria begin to flourish and build in numbers progressively. The ammonia from chloramine won't last long. In fact, you'd have to add more ammonia to meet demand pretty quickly. Fishless cycle = regular addition of ammonia to around 5ppm. This eventually requires daily dosing.