The most repeated conversation in a kitchen is “this is under-seasoned”, said for six months to somebody who cannot yet taste the difference at that concentration. That is a threshold, not an attitude, and no amount of telling fixes it.
About the numbers
These concentrations are ours, chosen so that two cups are tellable apart at a stated difficulty. The guide states 5–8% for a wet brine and 2–3% for lacto-fermentation and says nothing at all about seasoning, so nothing here is the guide's figure, a statement of correct seasoning, or a house spec. An eye chart's letters are chosen to be legible at a distance; the chart is not an opinion about what you should be reading.
And the honest limit: this measures your palate only if you build the cups truthfully. Weigh them, mix them fully, and have somebody else pour if you can.