This is my theory about iron test kits:
According to Karen Randall:
“At the same time, do not be fooled into overdosing your tank if you do not get an iron reading in the first few weeks of supplementation. Plants can store quite a bit of iron. If they have been iron deprived for a length of time, they will suck up all available iron until they have stored as much as they can.”
I think this is one of the reasons that people do no trust Iron test kits. You can be adding iron and test results show that there is no iron. Plants a sucking up all the iron they can. On the other hand you may not be adding iron and the plants are doing just great. This is when plants can outcompete algae. If there is no iron on the water column there will be no iron for algae and plants will be using the iron on their reserves. The goal is to have zero iron on the water column and plenty of iron on the plants reserves. You should add iron only when plants starting running out of iron, so they will consume the entire iron on the water quickly. As experienced hobbyist, you know how plants are starting to show iron deficiency symptoms and you correct it. For you, the iron test kit has no use as iron on the water column should be always zero.
So, to test an iron test kit, do what Rex suggests: test it in a bucket.