IME, all three N products (NH3/NH4, NO2 and NO3) do read positive despite being bound by the Prime....
I tested this at one point using the API test kit.
I don't have any tanks with ammonia or nitrites to replicate now, but as I recall ammonia was affected quite a bit. Don't remember now how nitrites tested. Just tested nitrates again since I do have a tank where I know they are relatively high.
Top tube is without Prime (I'll qualify that some in a minute). Bottom is with .1 ml added directly to the tube prior to testing and letting it sit for a few minutes.
You can see it more clearly in person than it the pic but still should be able to see a difference.
The qualification that I mentioned above re the first tube is that I tried adding Prime in .1 ml increments afterwards to see whether it would change after adding reagents. Does not appear to after a full 1 ml added over time and waiting quite a while. (That tube also is further diluted in color by 1 ml as a result which accounts for the greater volume seen.)
It's been a long time now but doing the same with measurable ammonia, seems to me that it was affected both before and after adding reagents. If I remember right, adding a couple of drops of Prime to a test showing relatively high ammonia changed it to show little to none.
Granted testing like this by adding Prime directly to the tube is at a much higher concentration vs in the tank but I was going for a test which would clearly show ANY effect. Whether it would make a practical difference in normal testing results I don't know. If someone has a tank with ammonia/nitrites (or ammonia available otherwise), they can test in the same way. I meant to do that at one point when setting up a new tank but never got around to it.