Let me add a correction here:Except, unless I’m mistaken, ICP doesn’t test nitrate. All ICP vendors use a colormetic or titration type kit just like us.
In that case ICP didn’t out perform a hobby kit. The ICP company’s “hobby kit” just out performed your “hobby kit”
We do measure nitrate with anion chromatography - the gold standard for laboratory nitrate measurements, that does not suffer from interferences (eg from nitrite).
Unfortunately the misinformation that laboratories are using "test kits" to measure nitrate is hard to get rid of ;-)