Call me skeptical. Before I'd dump on a major manufacturer (or 2) I'd ask these questions.
1) Did you do the test correctly? The nitrate test above all the others has much more laborious instructions on preparing the two solutions. If not mixed properly over the course of using the test the solutions can become not mixed in the proper ratios (in each bottle) thus making all the tests going forward from that point suspect.
2) Has the test kit expired?
3) Are the test kit solutions still viable
4) Did you test known samples? 1 vial pure RO
; 1 vial pure RO
+ a known quantity of nitrate ; 1 vial pure RO
+ a known quantity of nitrate + Prime and compare the results?
5) Did you dose the Prime correctly?
I also am not clear where you drew the generalization from ammonia and nitrite from. Prime 'locks' ammonia by changing it to ammonium (which is not toxic to fish). You would have to know if your test kit is testing for ammonia and ammonium or just ammonia to gauge your results based on that information.
The nitrite claim I've never heard.
Prime has been around for a very long time and has IMO
an unsurpassed reputation in the hobby (I would venture to guess in the other disciplines as well)
Your link above seems to speak to Chloramine, which isn't ammonia nor ammonium.