Hi Stuart,
I think what is meant is that 'supérieur' and 'inférieur' are comparative in themselves, so you would not say 'plus supérieur/inférieur à' , as with normal comparisons highlighted by Chris .
The important bit is that you use 'à' afterwards and not 'que'.
Hope this helps!
The usual way you form a comparative is with "plus" or "moins".
Il est plus âgé qu'elle. -- He is older than her.
Marie parle le français moins couramment que moi. -- Marie speaks French less fluently than me.
Here is a link to the corresponding lesson: Plus ... que/Moins ... que/Aussi ... que = More ... than/Less ... than/As ... as (Comparisons with Adjectives in French)
-- Chris (not a native speaker).
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