I don't think the test is good. BGA is better
Sure, test kits are fallible, but they get an unnecessarily bad rap.
I have blamed my test kits for being faulty numerous times, whenever what they told me didn't fit with my understanding on how things work.
Now that I'm more experienced, I look back on all this and find my tests have only lied to me once - an expired KH test that gave readings so high it was immediately rejected.
BGA consumes an enormous
amount of nitrate. So much that it can cause levels to bottom out, starving your plants, which is obviously a bad thing. The BGA does not starve itself in the process, because it can directly utilize additional nitrogen sources that plants cannot.
So you add more nitrate. And sometimes the BGA goes away. Folks say nitrate kills BGA. I say if BGA loves nitrate so much, all you're doing is feeding it.
What's important is that now there's some nitrate left over for the plants. It's the plants that beat the BGA, not the nitrate.
But they may not be able to fully accomplish this feat, if there are nothing but slow growers. Or few plants. Or anything else affecting plant health. Or areas of a tank that act as almost independent ecosystems, isolated by poor flow or physical barriers.
So I believe [thefisherman] and his test kit. It's no surprise to me that additional nitrate didn't solve his problem, and that results for increasing nitrate are so varied in general.