It is oddly used there and difficult to interpret. 'On the spectrum' usually means 'more or less' or 'within a/the range of possibilities or gradations', but only context will reveal whether it means a lot or a little, relatively.
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SuperESLon-the-spectrum inabilityI interpret this as "noticeable inability"; "obvious inability"; "perceivable inability".
SuperESLWould you say most literate native speakers would have no problem understanding the meaning of the phrase? Or would you say this is an idiosyncratic phrase that would give most people pause?The latter. Idiosyncratic.